> Nothing but their ethics.
That's not something a company the size of Apple can count on.
> And why would someone who's willing to sell bugs to criminals bother with a site that's already been picked over by bug bounty researchers?
Because it's Apple, it's one of the biggest companies on earth. iPhone jailbreak vulnerabilities alone fetch millions on the black market.
If you know the bug bounty program doesn't pay much you can expect only the trivial things to have been found, and if you're very skilled you know you still have a good chance of finding things to sell.
> And lastly how would paying more for bugs prevent someone from also selling it to criminals?
It would keep more honest people interested in your bug bounty program instead of doing something else.