This was pretty blatant. The professor told them they couldn’t do this outside the free speech zone, which is obnoxious enough… except the university doesn’t even HAVE a free speech zone anymore. Current policy said the students could do that. It’s possible that the professor’s knowledge was a few years out of date, but they also told the professor that they had specific permission, and he couldn’t be bothered to verify that, but kept erasing. The professor not only erased messages himself, he told students (allegedly from a class he was teaching) to help him remove the messages. Depending on the exact timing it’s possible that some of this happened during class time, or at least that it caused the students or professor be late for class.

As a bonus affront to the First Amendment, the students helping to erase the messages said repeatedly that recording them doing this was illegal. Maybe the professor can’t be blamed for that, but he sure didn’t go out of his way to correct the students. More bonus points for the professor claiming that defacing the messages was just his free speech.