If I moaned about my job/customers in a way that I was personally identifiable - I would be sacked, because it's wildly unprofessional. And I just work in an utterly pointless shop. So no, you aren't allowed to just 'post what you want', or indeed, you are, but you have to accept you might pay a price for that. Even more so when your job is actually of serious importance like NHS workers. We all get fed up and have a whinge, but you can't be running your mouth on a public platform about the very people you are entrusted to protect. Regardless of what you think about their life choices.
Alot of people seem to be ok with judgement when it's towards other people, but it won't stop here. What about when you are judged for a choice you've made, by the people who are supposed to be caring for you? This is why we don't do it. None of us are perfect, we are all liable to judgement.