Is that a principle you'd be willing to enforce across the board? A professor lets his "personal political opinions interfere with classroom teaching" and therefore he should resign?
Would we have any lawprofs left? The lawprofs would all say: It depends upon the meaning of "personal political opinions" and "interfere with classroom teaching." Either these weren't "personal political opinions," or, if they were, they did not "interfere with classroom teaching." And: The notion that you can sanitize all politics out of teaching is delusional. Unless you hire the shallow, dull professors who don't think about the real world at all, you will only push professors to hide their political opinions under a veneer of neutrality, distorting everything.
(I'm not saying that the professor in question — at the linked article — shouldn't be reprimanded in some way, but he should not lose his job. We professors should all be better at using classroom time to give students the best education we can.)