I don't want to speak for the motivations of others. I agree, that if the only reason a PSU fan doesn't want to support him is that he is representing Mexico and not U.S., it is pretty despicable given what he did to help the program. A perfect example is Franklin Gomez-I hope all MSU and PSU fans (and NCAA wrestling fans in general) supported him to some extent.
However, there is a legitimate reason to dislike the way the rule is constructed because it allows athletes to quickly switch nationalities with minimal connection to the country. In my view, the Olympics are meant to represent the best of each country in competition. If an athlete can't make the team for their actual country and then starts to look for an old familial connection to make another team, it takes away from the credibility of this being a competition between countries.