The general public does not understand federation. When Threads makes content that I have created via kbin.social visible on Threads, very many people are going to think that that content was created on Threads. And Meta then takes that content, aggregated with all the other non-Threads initiated fediverse content, and monetizes it. They are using "not their content" to enhance the desirability of their portal, and certainly placing ads in its vicinity. As with any instance, they can also curate that content to promote their chosen agenda, which is surely in part "increasing engagement."
We've seen how "increasing engagement" has been done by Meta and other companies already: ragebaiting and misinformation. While there is no way to completely prevent this, I want to avoid content that I have created from being used in that way. If there was a way for me to individually defederate from Threads, so that Threads could not see my content, I would turn that switch on in an instant. So far as I know, the only way for my content to be excluded from being viewed via Threads is for the instance my account is on to defederate. I'm not in any way asking for kbin.social to defed from Threads, just noting that that is currently the only functional way to accomplish the stated goal.
I do understand that there are already instances that have done that very thing, and I am certainly able to jump over and use one of those instead. I may do that at some point, but I am pleased with the interface at kbin.social, and developer of kbin's work. For the moment, I want to watch and see how things play out, becoming more informed before I make a decision about how I interact with the fediverse.