To add an opposing anecdote, after 30 years in the biz I've never seen this. Not once.
Also doesn't the contract has something about the number of hours? It may not be enforceable, but it's on paper.
It's totally cool that we as professionals discuss our commitment and get a flexible working arrangement. It's not cool to say you will be available to respond to incidents and what not and then not do that.
It is not their concern what I do on social media. It is unfortunate that there is so much power imbalance that most employees are afraid to criticize their employers in public.
It is purely on the employer's end, and I cannot find a legal restriction against it in any state. So it would only vary by employer.