This is legal to do without written permission? $6,500 for this feels like peanuts. The potential reach of such a hack is insane, especially with access to Github. OAI is lucky they were ethical and didn't sell this for several hundred thousand to a malicious third party.

$3500 when you consider they returned $3000 of that back to OpenAI in the form of burnt tokens.

It depends who you're hacking, where they're based, where you're based, and what you do. If you're extremely careful not to break any of the rules it can be completely legal, as it was in this case. Many jurisdictions make it completely illegal. I agree that $6,500 is a pittance.