If the rule is stated at the beginning of the bounty I see no problem, it is the same as kyc, it is each hunter own right to join or not... and the same applies to the manager, they are within their own right to impose the rules they think are best fit for their campaign...
I would not subscribe a bounty like that because it seriously limit one's freedom... I agree that there should be rules in place to avoid brust spamming, but limiting the number os counted posts per day is enough, I think