I am not trying to speak for pg, so only take this as an advice of an entrepreneur:

Incubators like ycombinator have seen thousands of start-up ideas over the years. Most likely some of them overlapped, and there has probably been a good amount of duplication (deal of the day site--only for food!). If there was an NDA for every single one of these, any successful company that might have duplicated an idea unintentionally (ie, they came to the same idea themselves) might get in trouble later down the road when another entrepreneur says "hey, that was my idea, they must have stolen it!"

EDIT: seems like me and iqster had the same basic comment :)