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.
Bounty Hunter is also a job. People promote projects and do it for tokens and not for real money. The only disadvantage of this work for the money for it you can get in half a year or even a year
Time is money, promotion of the project is also work and the project rewards people, everything is fair.