I agree that it is an expression of free speech, but universities, who employ these academics also have the right to fire them. Anyone has a right to speak about whatever opinions they have, but in doing so, if it hurts their reputation or employability, they ought to suffer the consequences of it (others exercising their freedom of expression in response to yours).
That said, however, it is upto the universities whether they wish to teach politically neutral courses or not. I am sure there are some arts colleges in the States that have strong left-wing reputations, and on the other hand, business schools may have a strong right-wing streak - and if they are renowned for it, it may actually be a desirable reputation! In Australia, such specialisation and diversity of tertiary education doesn't exist, so the political bias in a university is either a neutralised by a tug-of-war or diluted by the apathetic departments.