Meta can do whatever they want with Threads, in the exact same way that mastodon.social is it's own independent instance and can do as it pleases.
If Mastodon users want to follow or even see content on Threads, it's entirely optional, depending on your current instance's stance. You can always move to another instance that shares your views (and there will be plenty that do) and has defederated from Threads, or you could roll your own instance and be in control of what instances you interact/federate with for yourself.