They're trying to build in Activity Pub so Threads users can connect to Mastodon etc in an opt-in way.
Nothing that would be seen as scary if any smaller actor were to do it. But because it's Meta and Meta is genuinely evil a lot of the times, people automatically assume it must be done with the worst of intentions. Which is a reasonable suspicion to have, but imo shouldn't go as far as pre-emptively pronouncing anything they do as obviously evil, when the actions itself give no such indications yet