I guess lots of people don't read employment contracts but it's actually quite common to have clauses re: disparaging the company etc. it's not something particularly unusual.
AFAIK in this case though the issue wasn't so much the criticism/feedback but rather that they were badgering other employees to sign a letter, fill out surveys etc.
I get it makes for a neat/simplistic argument along the lines of "Ha Elon, you say you value freedom of speech but didn't you just fire employees for exercising their freedom of speech" but it's got little to do with the ability to publicly air views on a new "public square" like Twitter and treating people fairly there.