It's fair enough for companies to set their own rules for internal communication, after all not every internal board is a public forum. The issue here is obviously the particular form of moderation, i.e. bosses trying to shut down labour organizing.
Over a chat app they provide for work use, I'm not seeing the ethical issue. Pay and overwork is one thing but why can't these employees use their phone for things they don't want the company to observe? Phones are cheap.
People are living social creatures, they are not robots, and the pursuit to treat humans as APIs for units of productivity will lead to a predictable outcome of causing human misery.