[tych0]

I doubt it. Why wouldn't github want to pay more so that black hats also sell them bugs? Indeed, these are the very bugs that are going to be exploited, so it makes perfect sense for them to pay whatever it costs.

The real reason is probably: because nobody else does. I think it is doubtful black hats would sell their bugs to github unless github was paying 2-3 times the market rate, since the black hat can sell the same bug to multiple people.