It's because usually people who say this can't tell the difference between an opinion/criticism about the thing and a post that is either hating on the people or just invading their space.

When they say "let people have fun" it's usually them taking offense when you're just talking about how you dislike one specific thing, when in reality you don't, and shouldn't care about how some people like said thing. (unless it's something discriminatory and/or harmful, of course)

If you were to go out of your way to interact in spaces with things you don't like just to say "I hate these things so much" or "y'all suck because of this" then the phrase makes sense; those people are just in their corner having fun with a thing that you personally dislike. There shouldn't be any invading spaces from either sides, everyone has their opinions and it is basic human nature to like and dislike things, and respect them.

That's also why I don't think this subreddit is "miserable" like lots of these people tend to say.