Bounty programs do not attempt to compete with black hat markets or outbid black market rates.
The purpose of bounty is to encourage white hat hackers to challenge one specific application instead of millions of other applications out there that the white hat hacker could spend his/her time on.
So it's saying "Hey...instead of working on that random application why don't you try to hack us because hey you could earn some money too".
It's assumed that the person is a white hat hacker who would not sell the bug in black market anyways, even if there was no bounty.