If you were the only party to win in the spirit of the competition, you should be the first person to accept the prize money. Why would you let the unethical companies get even more prize money from a hijacked idea? Also, if you make an ethical stand too early, you lose the ability for the Hackathon to publicly list you as a winner and give you a platform for reform. That publicity will create a situation where other individuals reach out to you proactively and you can tell your perspective as a winner.