> If the behavior on their platform is not disruptive to it, then what precisely are they defending their users _from_?

From being surrounded by Adams. Either you act in cases like that, or you become known as "that place that likes to invite Adam (and others)". And it's completely up to the host which side they want to be on. Twitch chose to end the invitation.

> Invite someone on the most wanted list.

That's not even close to the case discussed here.