It's all in the moderators. They could delete meme posts or even filter posts and have some guidelines they stick to. Also, the “big” groups are almost all owned by the same cabal of 10-20 “super-moderators” that do it only for name recognition. I got pitched out of like 5 subreddits because I posted 1 comment in an anti-vaxxer sub to rebuke an obviously wrong anti-vaxxer rant. They didn't care, and after I cooled off I didn't care either when I realized those subs were all garbage anyway. I also sent them all strongly worded and curse filled responses to the most prominent “super-moderators” to let them know they are all garbage for blanket bans. One of them tried to get me pitched off Reddit and a Reddit employee that I know said the people over at Reddit couldn't care less about what moderator opinions are, and they don't really have any power outside their subs.