Hm, sounds like a quite good reason. Good point.
But you agree that there is no need to have more than 5 accounts for sure? The persons who participate in the bounty campaign with multi accounts, they most likely have more than 5 accounts, and they are posting from all 5 thus increasing the spam level on the whole forum. I would say that technically possible to create limitations based on MAC/Hardware address, for instance, there are 2 accounts allowed per one device. The business people will be happy enough and bounty-cheaters will be down.
If that's the only thing that you want, then give the merit system time. Bounty cheaters will not get their accounts ranked up with merit system now and because of default trust members who are on the move to track merit abuse, they will get red tag those giving their alts merits or those who are into buying and selling of merits. Also, a default trust member can give them red trust if they are proven cheating a bounty campaign with multi accounts. I think it is enough countermeasure for that.