My impression is pg has banned some sources manually:

Huffingtonpost.com - for linkbait blogspam.

theonion.com and buzzfeed.com - off topic entertainment.

gawker.com and its other domains - general sleaziness.

pg might want to consider unbanning Buzzfeed, though. In the past year or two they've added tons of reporters who do pretty decent long form articles you would expect to see on Slate, The Atlantic, or Forbes, and the risk that HNers will upvote "The 50 Greatest Cat Photos of All Time" is probably pretty low.