Considering exploits are supposed to be hard to find (why they're large bounties), it's just the incentive/hush money to pay the hacker, because you have to consider a few things...
1) why and how did you find the exploit (were you trying to hack someones account, stumble upon it [that's lucky], are you a security firm [meaning you have success in this before], were you black hat contracted, ect).
2) a hacker would prefer the recognition [possible employment], the reward [sandwiches aren't free], and release of liability [a company may still file charges for probing their systems 'weev,' is an example]
I can think of very few vulnerability testers that have gained employment at the companies where they find the exploits. Comex is one I can think of off the top of my head (created the jailbreak for iphones, landed an internship at Apple, then career Google).