Most of them are unable to. That's because they don't have such in prior or are insufficient to handle the whole marketing expense alone. Also there are other things for which they also need funds to in other operations which they give more priority over marketing expense. Reason being they can't settle payments with tokens in such operations. It has to be done in stable currency or some other form of payment which has a value at that point.
Thats right and that will be an obstacle faced by most altcoin project developer. The bounty program launched aims to gain a wider marketing reach among potential investor in the forum. I think the developer will have difficulty paying the cost of the campaign if they have to prepare initial capital for all participant unless their token or coin have been traded on the exchange and they will pay it every week or month with token or coin with or without escrow.
I think OP advice lead to the frustrating form of most bounty hunter who are already very vulnerable to promoting bounty without getting paid. The lack of funding in the last few year on the altcoin project has caused many project to fail and must make participant unpaid. Of course it really make the campaign participant disappointed.