They already do incentivize white hat exploit efforts[1]. The author seems to have run afoul of one of their rules[2] by impacting other users, but I don’t think that impact could be knowable without trying.
GitHub could trivially honor the request without changing the incentives or even taking any defensive implementation action, by specifically citing this experiment in the rules and maybe adding some more specific wording to the TOS.
1: https://bounty.github.com/
2: https://bounty.github.com/#rules