The Olympic movement originated with a strict prohibition against professional athletes, which was still in effect in 1980. Hockey players employed in the NHL (like professional athletes in the NBA during the summer Olympics) were prohibited from participating in the games. For a long time, non-communist countries complained about what seemed to be professional athletes from communist countries, but since the later could maintained that everyone was guaranteed an income, the "capitalistic" designation of "professional" athlete was meaningless in those nations. Ultimately, it made it nearly impossible for young teams composed of college athletes to defeat the hardened veterans of the Soviet team, making the 1980 gold medal for the USA all the more remarkable.

So he US Olympic Committee, and indeed the Olympics at large have had a long standing tradition of amateurism. The US just held onto it longer than most. In particular until the 70's the Olympics forbid athletes from seeing any direct payment for their performance. The rules went so far as to even ban those who had served as any sort of coach or instructor in their sport.

However this was easily gotten around, and everything from trust funds, to under the table cash awards, to what was most common for the Soviet nations was enlisting your best athletes in the military and then paying them simply a soldier's salary. However it was with the understanding that their job was to play the sport.

Its why for instance the Red Army's main hockey team out of Moscow ended with a record of 24-8-2 against NHL teams by 1991.

In the post war years the Olympics were greatly influenced by the management of Avery Brundage, who had rising from management of the USOC to the IOC as a whole and was a fierce defender of Amateurism. And while the early 70's saw an easing of the regulations, after Brundage's retirement, it was a gradual process, and the 1980 US Olympic trials were still conducted in a spirit of Amateurism barring pros, meaning college players were the highest quality pool they could get.

The US had also been burned by running afoul of Amateurism rules in the past in other sports. Most notably, Jim Thorpe, perhaps the single best American Athlete of the 20th century, had his Gold medals for the Pentathlon and Decathlon stripped from the 1912 Stockholm Games. It later came out that he had accepted money for playing in a few Baseball games years before and thus was not an amateur athlete(though this was a common practice for college players at the time most used aliases). And it was only in 1983 that his medals were restored, in part on a technicality on how they were stripped in the first place.

But regardless after 1988 all athletes of any level were then welcomed to compete in the Olympics.