As I understand it, VCs routinely refuse to sign NDAs, because (as mentioned elsewhere) it's very common for two teams to independently come up with the same id

As I understand it, VCs routinely refuse to sign NDAs, because (as mentioned elsewhere) it's very common for two teams to independently come up with the same idea, and the VCs don't want to be in a situation where they have to convince a jury that they didn't reject a certain team's proposal and then leak the underlying idea to someone else. (And yeah, I'm cynical enough to assume that once in a while that's exactly what happens.) So in this respect YC is no worse than anyone else.