@aral
So, it occurred to me last night that it is possible that the Mastodon Organization cannot block or even encourage a block of Threads for the same reason Twitter can't ban them: it could potentially run afoul of anti-competitive laws. Because of this, legally speaking, the best course of action is to let them federate, then make a big public paper trail of the problems associated with federation, and use that as precedent to start limiting interactions. In short, waiting to block may be legally necessary.
That said, realize that everyone who has made the decision not to block has done so after some clear thoughts about it. We are aware that this is a bad actor, and we are aware that we will likely have a moderation scenario the moment they actually federate, assuming they even do. We have weighed that against the cost of preemptively blocking such a large new entrant, and decided to hold off for the moment, and wait until there is an actual problem.