These arguments always remind me of people claiming that certain professions are not paid enough. They forget that there is a market for labor and in this case the labor is finding vulnerabilities. People will either be willing to work for the posted price or not. In the case of pen testing facebook I'd be willing to bet there are plenty of people out there looking for bugs who aren't even really concerned with what the final payout is going to be.
Yeah, they could have gotten completely owned if he didn't report this. But to him reporting it and getting 10k in compensation was sufficient. Why would facebook pay him a million if he was willing to take 10k?