I'm going to tackle your question in a few parts, as there's a couple of threads that start distally before converging. I'm going to focus more on the Olympics themselves than on individual sporting bodies as that it where I can better speak on it.

##The 'Purity' of Amateurism

Let's start off with a fun quote from my friend and yours, Pierre de Coubertin:

"The most important thing in the Olympic Games is not winning but taking part; the essential thing in life is not conquering but fighting well."

Coubertin, being the founder of the International Olympic Committee, was an idealist that was in love with the trappings of the European aristocracy. In particular he was drawn to the English upper class and what he viewed as an 'indifference' to athletics. The quote I provided above encapsulates not just what Coubertin saw as his ideal for the Olympic Games in general, but in life period.

First, you are definitely correct in your assumption that 'purity of sport for its own sake' was primarily centered around being a class issue. Coubertin, being an aristocrat himself, came from a highly privileged background where athletics was not a means to a potentially higher station in life and thus should be celebrated for its own benefits - if some of these benefits were from keeping idle hands busy and not following 'sinful' pursuits all the better. When Coubertin was introduced to English public schools he saw the ability for athletics to contribute to, "moral and social strength." Strict adherence to amateurism ideas was a method to these ends, as the athletes would not be corrupted by outside influences - be they political or monetary. When combined to his romanticism about the Greeks, which we will get to in a moment, he saw athletics as an endeavor that could cross nationalistic barriers.

This also partly influenced why Coubertin saw the games as something that should be apolitical and outside of the influence of other countries. To once again quote Coubertin:

"The Olympic Games are for the world and all nations must be admitted to them. [The games exist not for national sentiment but] for the exaltation of the individual athlete."

A second, partly nationalistic influence (which I do not believe Coubertin ever saw the irony in) towards promoting athletics and amateurism for the sake of athletics is that he believed that a nation that had a strong basis in physical education would breed a stronger military. As a Frenchman this was important when viewed through the lens of France's humiliating defeat in the Franco-Prussian War. This view of a strong foundation of physical education promoting a nations military strengths was tempered by the frankly aristocratic way of looking at athletics: as an endeavor of the upper class for its own sake. Coubertin saw organized, amateur athletics as a possible way for nations to subvert war as it would allow the upper classes from disparate nations to compete against each other and form ties. If it would also allow 'lower born' individuals to compete with their 'betters' than that also would be encouraging in strengthening international ties.

The question is, where was Coubertin and his like minded compatriots drawing these influences from? After all, the ideals of amatuerism weren't invented by Coubertin (though he did much to promote and shape them) The answer is because of a misguided view on the Greeks. Coubertin and his ilk idolized the ancient Greeks, while also grossly misrepresenting their views. The Greeks represented to them an ideal of civilization, and one in which they projected their own views onto. One of the key aspects they 'drew' from the Greeks was that of a strict code of amatuerism. I believe David C. Young summed up this view far more concisely and eruditely than I can hope to by calling this:

"...a far-flung and amazingly successful deception, a kind of historical hoax, in which scholar joined hands with sportsman and administrator so as to mislead the public and influence modern sporting life."

Young's work, although contentious on some of the conclusions he draws, does well in representing consensus that Greek athletes of the type that Coubertin idolized were far from strict amateurs who competed for the sake of athletic pursuit. While at the beginning of the Olympic games there is debate on the true nature of the material rewards for competing, the evolution of the games of the ancient Greeks led to extensive material rewards for competing. This evolution in itself would play out in the modern games.

##The Decline of Amateurism

From the beginning of the games there was a slow erosion of the ideals that Coubertin attempted to maintain (the inclusion of women in the games flew directly against the aristocratic views that he held). The Cold War would be what finally killed the concepts of amateurism, but not without compromising what were originally seen as Olympic ideals. American Avery Brundage - while holding many of Coubertin's beliefs - would ironically lead to the slow chipping away of all of the ideals in which Coubertin would found the games.

Brundage was as much a believer in strict amateurism as Coubertin was, and ruled over the amateur code with an iron fist. To quote,

"The amateur code, coming to us from antiquity, contributed to and strengthened by the noblest aspirations of great men of each generation, embraces the highest moral laws. No philosophy, no religion, and preaches loftier sentiments."

Brundage sincerely held the belief that professional sports were of a lower moral character, and his rulings on amateur status reflect this. He believed that amateurism was as much of a mindset as it was a rule, and thus if you ever accepted material gain for sporting endeavor of any kind you had permanently forfeited your amateur status across every athletic pursuit. He also was a firm believer that the Olympics should be apolitical - famously refusing to allow the American delegation to boycott the 1936 Games in Berlin as it was a political sentiment that went beyond the scope of the games (even though, famously, these games were expressly being used for political propaganda by the host nation).

With this being said, Brundage's tenure as president of the IOC would see politics increasingly infest the games. With the URSS team being not just state sponsored, but non independent of the government, it flew directly in the face of what the rules of the game were supposed to be: amateur athletes competing not for nationalistic pride but for the sake of athletics. The idea that Soviet athletes were amateur was also pure fantasy: most were officers in the Red Army, who were conveniently ordered to focus on athletics. The Olympic ideals of inclusion would win out, however, and the farce of Eastern Bloc amateurs would become pervasive.

The fantasy that the game were for athletic celebration died a swift death with the worldwide realities of the time period. The advent of television turned the Olympic Games into a showcase of propaganda that television was more than willing to take advantage of. The business world saw dollar signs, stories needed to be told, and political realities would play out. The Americans, and most Western nations, would continue to send amateur level athletes to the games to compete against Soviet officers (this is why the 1980 'Miracle on Ice' was seen as such a big deal), but with the games being sold for profit on television and becoming quite lucrative athletes were getting sick of being used as political pawns for a life of poverty.

One of the most famous athletes to call this existence into question was Bruce Jenner who openly told anybody that would listen that the only reason he was competing was for the fame and fortune success would bring him. The fact that when he won gold in the decathlon at the 1976 he immediately retired and walked into a world of paid product endorsements, television appearances, and wealth was the first overt direct shot at this, by now, loose concept of amateurism.

Finally, the 1992 Olympics ended the pretense. With the Soviet Union collapsing the Cold War exercises were no longer going to be played out. Instead, IOC president Juan Samaranch decreed that professionals could compete in all events. Why? Because it would be more attractive to television audiences. The games were to become sanitized snippets of human interest stories of professionals competing for nationalistic pride, instead of Coubertin's vision of amateurs competing for athletic's sake.

Sources and further reading:

• Davies, Richard. Sports in American Life.
• Guttmann, Allen. The Olympics: A History of the Modern Games & The Games Must Go On: Avery Brundage and the Olympic Movement.
• Young, David. The Olympic Myth of Greek Amateur Athletics.