FIrst, yeah, professors can be asses.
And, contrary to popular belief, there are plenty of racist, fascist or otherwise scary professors out there.
And, well, sure, academia in the US is predominantly left leaning. Why is that?
Well for one, you ain't gonna get rich teaching in a university. Many of those who espouse the ideals of the party in power, such as cutting government spending for education and pure research, while encouraging corporate profiteering of the public sector seem to be drawn to the world of business more than academics. (that also means you'll get a significant number of libertarians and similar economic freaks in universities too)
For another reason, there are plenty of privately-funded think tanks that hire conservative academics. Capitalism in the US isn't simply about the free market, but rather about getting the government to create the legal conditions most favorable for a given corporation, and least favorable for its competitors. That requires a multi-pronged approach, and one of those prongs is para-academic work. So there's plenty of funding out there for academics who want to shill for the corporations.
Which brings us back to what academics should and shouldn't be allowed to do. Militaries exist to support the state, and, given their very real power, many safeguards must be in place to prevent them from opposing the state. That's why there are restrictions on what militaries can and cannot do. They are tools in the hands of politicians. Otherwise, we get a military dictatorship.
Universities exist for the benefit of the citizens, and they do this in several ways. First, they provide education, and not simply vocational education (aka "learniing to slave"). They should provide the citizens with the critical apparatus necessary to be informed participants in the democracy. Second, they provide a space where people can expand the bounds of human knowledge.
The moment we say, "and professors must support the state", we kill both these objectives. The very first lesson a university should teach is to question authority. That authority includes both the state and the professor. We don't want you to think like us, we want you to think for your bloody self, thank you very much.
And the moment we make academics answerable to something other than the boundaries of professional conduct (=not stealing other people's work), we damage considerably their ability to expand human knowledge. If you doubt this, google "soviet genetics".