I believe defederating should be a user choice rather than an instance unless done for spammy/toxic instances. If instances starts to be too liberal with defederation, you create silos and introduce more hurdles for the growth of fediverse. This creates a broken up network that may not be social for everyone.
Obviously, you can be on an instance that defederates Threads.net if that’s your preference.
Sure they are unethical. But the millions of users who have joined them are not and they are not that tech savvy to have even heard of Fediverse. Federating with them opens 2 avenues: Possibly decrease the influence of X/Twitter as it gets more toxic and introduce general people to the concept of Fediverse and give them an option to easily migrate to one of the better Mastodon instances in future from Threads.