You don't know that they haven't faced abuse like this before, you're just assuming this is the first/only case, maybe this is just the final straw.

But that's also moot - people don't have a right to abuse others, people have a right to not be subjected to this kind of abuse.

If a person is not allowed to halt a project when they receive abuse, then you're saying that they are required to accept abuse as part of their (typically) unpaid labor. There is a huge difference between "I am willing to put up with this" and "You are required to put up with this". You are saying that because the former applies to you, that therefore the latter applies to everyone.

I get that this is the long term outcome of the "don't feed the trolls" concept, which is tacitly the message "being a troll is a-ok", which has led people like this ass to think that just because they can send this kind of shit, that it's ok to.