Icos look for bounty hunters to promote their project through the power of social media.Sometimes, some of these hunters also double as ico investors. But it looks like bounty hunters understand ICOs project and are able to determine very good projects before they invest because of their experiences in different projects.
Yes some bounty hunters are investors too. While doing a bounty at the same time they are also the buyers at ICO or after the coin get listed on exchange. The main purpose is to get more token/coins of the project.