This would break the way many users use Lemmy & Co. I'm happy about users that cross-post.
I only follow communities I subscribe to. Let's say I follow B but don't follow A. If a user posts something related to topic A and topic B only in a community A, which I don't follow, I would miss information interesting to me. When the user cross-posts to community B as well, then I'll also see that useful post.
Also, even the same topic/community appears on different instances, and some users may not even be aware of the same community in other instances. They would miss useful or interesting information without cross-posting.