Re "You don't get it, do you"
Errm, the contest is IMHO mildly pointless for a variety of reasons:
(1) Skillset. As with any pen test (which is what this is), it only proves that at a certain point in time a certain operating system in a certain configuration was (not) hackable by a certain person with a certain skillset, and who was or was not entirely awake at the time. That's quite a few variables that can change and invalidate the result so ever if you get an OK it may only be valid for a second.
(2) Return on Investment. You assert that the idea is to find new vulnerabilities (which, incidentally, are by definition "not yet known" :-). The question you forgot to ask is just how useful that public announcement would be to the wannabee hacker. Not only could he get exposed as "a danger to society, better make him a terrorist" - remember, there are laws out there that make security research legally dangerous, from a pure return on investment point of view such an skilled individual may turn instead to use his knowledge in writing toolkits for organised crime if he's not too worried about the police (or is pressed into "service"). So he's not going to show up on the radar.
And don't get me started on the risk factor of discovering a zero day publicly so that the supplier doesn't get a chance to fix the problem before announcement. Could become quite an entertaining liability problem for the organisers..