I think the issue here is more that our society in general selects these people for "success" more than those "without pedigree" by virtue of various complicated socioeconomic conditions. I don't think it's an active (or even "unconscious", if you'll permit the flawed adjective in describing a group of people) bias that YC has against any one group or another.

The simple fact is that it's easier for persons with these backgrounds to be less conservative in their entrepreneurial endeavors than for, say, the guy who struggled through a B.S. or B.A. program and who came from poverty. That sort of conditioning lasts for life almost every time. As a result, the population of people who are likely to put together a truly acceptable YC application are more likely to be found in the "pedigreed" group. It's a flaw in society at large, not necessarily with YC itself.