How did you get the idea they think anyone is being altruistic? Usually the complaint that people won’t vote in rational self interest is suggesting that people are voting based on irrational evaluations of their self interest: for the benefit of their unlikely future selves.

Public Choice theory is "a whole branch of political science" the same way "historic materialism" is though, with Buchanan instead of Marx, as it was created with the same kind of ideological motivations, with “state bad” instead of “capitalism bad” as the alpha and omega of the discipline. Interestingly enough, both shared the same contempt of democracy.