By federating with them, your instance is providing them with free content to profit off of. Every post you make is another post for their users to scroll through, another chance for them to inject ads even if you personally block Threads.
I agree with you. Fucking hate meta. Still, I think it should be a personal choice for users. But then again, lemmy is all a out choices and users can flock from one instance to another.
I think we might be mostly on the same page but to clarify: I believe that an instance admin choosing to federate with Threads is depriving their users of personal choice moreso than choosing not to federate with Threads as it's forcing users to opt-out their content being used by a for-profit company (by changing instances).
Immediate concern is difference in scale - we're a drop compared to Meta's ocean, and I don't see how we can have any shred of hope moderating the tsunami of content that'll be heading our way.
Long term is EEE. I have zero expectation that Meta would handle a union with the fediverse ethically, and that's their ticket to killing it off before it has the chance to grow into any kind of real competition.