I like your recap of the issue in a different comment, but I think you're not giving enough weight to the fact that it wasn't known before that YC funded the team starting on a different projects, not IM. In fact, even now this information didn't appear in pg's update, only in a comment of his later on. I think much of the vitriol you're stunned at comes from people thinking that YC knowingly signed off on and invested in IM's business model, and I think that's quite enough to be shocked by, if it were true.
That seems reasonable, and it's supported by IM claiming to be part of YC on one of their pages.
Still though, the response seems disproportionate. It's not like YC has recently developed a pattern of funding distasteful businesses (or teams); why did everyone automatically assume YC was behaving badly?
There still seems to be a problem in people jumping to conclusions before having enough information at hand. A lot of the comments on Aaron's case were symptomatic of this (the fallout too).
I'm not sure if this is a new problem here or not, but it seems to be getting a lot uglier recently.