There's no real explanation of why that isn't a good deal for an entrepreneur. You don't get to keep all that, but $500,000 dollars from YC for a 7-10% stake of companies (as others have said) with an expected value of $12.5 million is a deal that gives YC a maximum of $725,000 in value, given that 10% of $12.5 million is $1.25 million.
The question to ask is whether being part of YC adds $725k in value to your company; i.e., is it the cause of those startups being worth that much? Based on what others are saying in the thread, this seems to be true, but I'd love to know if people who were a part of YC felt like the assistance was worth that much.
My point is that it's difficult to believe that the average wealth of the entrepreneur is worse off by joining YCombinator even assuming all of the facts proposed in the thread are true.