Large corporations tend to put profit over morality and the legal machinery to even try to bring them back into line moves slowly (and expensively) when they do so.

If your Mastodon posts are visible from Threads, Threads may "accidentally" determine that that content was posted by one of their own accounts covered by their own Terms of Service and so choose to do something with it that the Mastodon user did not agree to.

Now consider that Meta, Threads' parent company has put things in both its Facebook and Instagram Terms of Service that say they can do whatever they like with content posted on their platforms, including feed it to any "AI" as they see fit.

And what about responses to Threads posts from the Fediverse? What rights do Meta have to those? I'm sure you can imagine what Meta thinks, or might allow themselves to think, in that regard.

Presumably you can now understand why some Mastodon instances (and other Fediverse places) don't want anything to do with Meta or their products and wish to protect their users from that abuse.

You might argue that it will happen anyway, but in their view, any protection is better than none.

And the great thing about the Fediverse is that you're welcome to find, or even run, an instance that leaves itself open to such "interoperability".