My understanding is that the traditional view of Philippe Petain is as a radical reactionary who became engulfed in the defeatist mentality of WWII France, and so he collaborated with the Nazis for personal gain and/or fear of the complete conquest of France.
However I read a few things describing the amount of respect he obtained from De Gaulle for willfully returning to France to face his punishment, and how even Truman desired for his release a year after the war ended.
It sounds like he's a chivalrous man. Just a leftover of the 'old-guard' of Europe, who was misunderstood by contemporaries.
Well, it's important to remember that Petain was also a hero of WWI in France and punishing him wouldn't have been a very popular decision for De Gaulle's government. Also, by the end of WWII Petain was rather old, turning 84 in 1940, further diminishing the need to punish him for collaboration (in this context "punishment" would typically mean execution for a functionary of his calibre).
For a lot of French people at the time Petain was also seen as the person who prevented the destruction of France. The fallings of the Vichy government were often associated not with him, but with the "bureaucracy" instead. You had this thing where people would say things like "oh, if only Marshall Petain knew about this, he would have fixed this immediately". This could apply to a wide variety of things, from dwindling rations, to corruption within the government apparatus.
At the end of the day, however, being old and a war hero does not absolve one of the blame entirely. Petain still presided over what was in effect an authoritarian regime that never had the legal mandate to rule the country, not to mention providing a certain level of support to the Germans.
That being said, Petain was not as pro-German as Pierre Laval (who succeeded him as the Chief of the Vichy government) whose legacy is (perhaps fairly) dominated by his decisions to go beyond what was asked of him by the Germans in terms of handing over French Jews (for example he chose to deport Jewish children under 16, while the Germans originally allowed them to be exempt, and he willingly used French police to accomplish this). Petain was rather anti Semitic, but he never went as far as Laval in this respect.
While Pétain's actions were often seen as grudging acceptance of German superiority where he would only provide the bare minimum that the Germans expected of him, all for the sake of France's well-being (which wasn't always true), Laval was seen as a more willing collaborator, one that truly sided with the enemy (also not entirely true). Laval, however, came to be blamed for almost every major transgression of the Vichy government (some of which should definitely have been attributed to Pétain) and was executed after the war, while Pétain was simply sent into exile (albeit one where he would still be kept in a semi-prison-like confinement).
A follow up question would be, why did Truman ask for him to be given asylum in the US? Wasn't this a French only problem?