It's not like YC gives them money and says "cya" and never talks to them again. They are probably forever involved in the start up until they exit, which doesn't seem to happen often.
That will reveal the false positives, but not the false negatives.
Exactly. We're painfully aware of our mistakes.
Then, why implicitly stating that "some" rejected applicants were bad persons? Why not simply saying "we did not invest because we don't think it will succeed."