Meta's content dominating the Mastodon feeds is actually a good point. With that said, the users on Mastodon, tend to flock away from Threads as it is. They are likely just to block threads as being tech savvy about this kind of thing.
However, some users might want to talk with their friends and families from the mastodon instance they are in and the fediverse was suppose to enable that.
So depending on what you expect will determine what server you'l join, whether your instance chooses to defederate or not defederate with threads.