What is the history of tenure? Why was it put in place to begin with?
I'm not sure of its origin, but the reasoning behind it is that it frees a professor to conduct research that challenges the status quo without fear of being fired. Frankly, given the present academic atmosphere, tenure probably protects non-leftists professors (if they can hide their views long enough to get tenure) more than leftists.
Note that Churchill is not being fired for his opinion. He is being fired for plagiarism and other related misconduct.