Bounty hunters are requesting to get paid through Bitcoin and Ethereum or other tradeable coins so bounty hunters will not dump their token when it hit the market, thereby protecting its price.
But they don't want that because they do not believe that their own token will have value or demand when it hit the market

and that's actually what's happening
That's one of the reasons I can think of if you have more post it here.
In 99% of the bounties, bounty managers have no role in deciding the payment method! It's the owner of the ICO who decides what to pay and how to pay. So your statement is technically incorrect.
Secondly, ICO is a funding event. If bounties are paid in bitcoin or any other tradable coins, then they would requires a huge capital to start with. It is cost effective for the ICO owners to use their own tokens to pay the bounty hunters! But I believe the entire bounty system is flawed! ICO owners should think about new marketing strategies!