Bounty campaigns are project that are made by ICOs, Bounties are build as an advertisement of such project to make their project successful and reach their desired pool. This project allows users to participate by posting and wearing the Bounty Signature and earn their Tokens as payment of their hard work.
You can't earn Bitcoin actually but it depends on what coin their advertising, and these coins are the payment for your participating. If the project collect a high pool then its a success and when its a success the coin you about to earn will cost high on different coin exchange and by that you can convert it to Bitcoin in order for to have a Bitcoin out of Bounty Program