There have been plenty of posts about it, and I can't find the one that better broke down the situation, but it went like this:

• We don't need Meta and they will be probably just after your data, which is anyway completely public regardless of whether you join threads or not (since the protocols are open, and they can simply look into everything you post here or there). So we might as well not give them data directly by joining their platform.
• the most our instances can do is to either federate with them, or not. But that's not the only thing, since you will still be seeing their content if your instance federates indirectly with them (through another one, that doesn't block them). This will leave 3 main groups of instances:

1. Instances that federate with Meta's Threads.
2. Instances that don't directly federate with Threads
3. Instances that block threads completely.

[1] and [2] will mostly carry on their merry way while [3] will be super isolated, since [1] and [2] will be the biggest groups.

I can't really recall the bigger arguments that summarise the idea, but it all comes down to:

• lets show Meta's users that they're welcome in the fediverse, and that they too can leave big corps for a more decentralised community. If we block them, some instances (that are maybe less liked) will instead be the front page of the fediverse for these new users, and that wouldn't probably make a good impression.
• Meta joining the fediverse voluntarily will be its own demise, since we are not leaving for them, and their users will see that Meta doesn't really bring anything to the table and instead just takes.