First, because there's no objective measure of benevolence, and second, because ranking VC's on a spectrum of benevolence vs. malevolence would strain the relationship between YC and these VC firms. You get a lot more traction making general statements like pg does here than you do by calling out individual firms.
Asking YC companies to crowdsource benevolence is hardly earth shattering. I'd be surprised if they don't already. The difference is offering this guide as a public resource. The risk is that firms could still treat YC companies well then treat others less well, but as pg notes, this type of duplicity is hardly worth the trouble.
