I think you're right that it's actually to do with attribution (i.e. They're not just watermarking your output as claude generated, but watermarking it as claude generated _by claude user id 73684_). I think it's more to do with the growing militarization of the internet. Just another brick in the wall of enter your phone number to create an account, send your id to prove your age, smile for the flock cameras, etc. Allowing the plebs to have privacy and anonymity is not allowed anymore.