Regarding Twitter, Elon seems to like trial and error, right? That's how SpaceX developed their rockets - by trying new things and testing them frequently, to see if they work.

So with Twitter I think he's just trying to see what he can get away with. And if he can't get away with something then he'll just roll it back.

As for Reddit, I guess they saw Twitter trying to squeeze more money from their platform and thought "let's try that too".