Participating in a bounty is a kind of investing. Bounty hunters buy investments for their time. Otherwise, they could earning the money for which they bought tokens.
I agree that bounty is like investin but there is one BUT. Investors usualy do not sell tokens for smaller price they bought them. Bountists very often sell the tokens after listing on exchanges independing the price.
I think a lot of depends from the project. But many bounties prefer to sell tokens even for the smaller price then nominal just to get someting.