I'm just guessing but I assume that, if you could query Google for a million tons of data about all the sites in the world, somebody would. Multiple somebodies would. Everybody would.
A Google API Key allows you to check your rankings through different interfaces (which IMO is not something they should ever take away from us!) but if you could use your API key to check out all the link relationships for your competitors, that'll run you up over the limit for your daily queries really fast.
Generally speaking, I think they'd rather you not exploit their resources like that. Vanessa?