They don't have to buy Bitcoin or eth or any tradeable coins because they can get it in their sales and if they didn't reach their soft cap they will not pay their bounty hunters, all ICO is like that they will not pay bounty hunters if they did not reach the soft cap
The bounty rewards are usually pre-set. What if in the end the sales result isn't enough for bounty payment? You want your goodness but project developers want their goodness. It's kind of a game theory. Paying bounty in ICO token is the fairest way for both sides (if that ICO doesn't scam).