Exactly what I was thinking.

(I think this is related to the bizarre phenomenon I occasionally see (particularly regarding the Topic That Must Not Be Named) of people saying they don't believe the conclusion of an argument because they don't think it will convince anybody – rather than because they're not convinced.)

So the group norm we really need to establish is that if you want to criticise someone for joining, only a solid case is acceptable; a cheap shot based on joining behaviour should reflect badly on the speaker.

Yes, but perceptions from outside the group are still just as problematic.