See here for PG's take on this:
From building spam filters to funding a spam company. Building things people want? Ha!
I guess it's worth noting other YC partners in that thread also defending malware, for example Garry Tan: https://news.ycombinator.com/item?id=5093746
It seems like the culture of spam and malware is deeply embedded in YC these days.
Edit: Oh, and Quora, even if that wasn't much more than YC lending its name.
The vast majority (but not all) of startups do dubious things. That's to be expected in an environment that glorifies "breaking things" and worrying later if what they're doing is legal or not. Being YC affiliated does not change that in the slightest bit.