That's pretty lame. Startups that show this behavior to the OSS community should be equally punished, be under the YC umbrella or not.
It's also false that YC will overlook IP theft for the sake of financial upside. One of the most fundamental principles that YC drums into startups is that they shouldn't breach software licenses. This comes from personal, painful experience for pg in the Viaweb days; a dispute over IP nearly killed their acquisition by Yahoo (or a major funding round soon before that). YC counsels startups to be super-prudent and upstanding about software licensing. Sure, sometimes startups will be cavalier about it and cross a line, in which case YC tells them to fix it immediately.