I also wonder if only the creator of the thread should be able to cross post it? Not sure if there is a reason why it does not behave like this at the moment - sure there is!
No, the creator might not know of all the communities where the post might be relevant. You’d end up either with everyone bugging the poster to xpost it everywhere; or everyone just copying the post, making things worse.
Seeing an xpost might lead you to finding nee communities you didn’t know, it’s one of the best discovery tools.