That's not exactly uncommon in the modern world. Especially with the prevalence of social media, expressing your opinion on anything publicly could cost you your job. Same thing happened to James Damore.
If you want to remain employed, you need to be quite cautious about what you post on non-anonymous social media. In fact, I'd advise most people not to post on social media at all.
This, on the other hand, is just about Amazon trying to control people to keep their poor treatment of employees hidden. There's nobody at Amazon who's saying, "Man, I'm quitting if those people keep trying to improve worker conditions here."
It is uncommon in the parts of modern world that have reasonably modern employee protection laws. Yes, most jurisdictions recognize that employers may expect a certain degree of loyalty from their employees. But unless you're a very high-level executive, just disagreeing with your employer's conduct is absolutely no legal grounds for termination.