I had a very visceral reaction to reading that YC had dirty money from spammers, but to be fair there's a much longer list of startups they've funded who are not airbnb-type scum (at least that we know of).
As the sysadmins say, Rule #1: spammers lie. If their "policy to forbid such actions" even existed it was a completely unenforced fig leaf of deniability, because that is in fact how their "person-to-person sales" contractors were allowed to spend their time until a third party did a little diligence and caught them at it, and unsurprisingly they're going to get away with it—there's been no mention of a lawsuit or anything to claw back what they got paid for poisoning the well.
What "evil" has AirBnB done? Yes, they're disruptive to the hotel industry, and you might claim that hotel taxes are a major and legitimate source of cost-recovery (although I'd be perfectly happy if AirBnB negotiated to collect occupancy tax and then pass it on to the city). That's approximately as evil as Amazon or any other online merchant, though.