A bounty hunter is expected to make through research part of which by trying to avoid bounties with many participants with a small budget of course such bounties will pay less at the end of their promotion however a lot of newbies never bothered they kept on joining as many bounties as they can to ensure that at least they will earn as much as they can not minding how worthless some of tokens will priced when listed in an exchange.
now you can see, when there are a good project and the likelihood of success and paying participants will be filled by bounty hunters. we cannot avoid that, especially if there are no rules on limiting participants from the bounty manager.
As for the bounty listed in the exchanges, I think they should limit the participants to make it possible for the bounty hunters to get a good amount of money from the campaign. I recently saw the IQ cash campaign, they listed at major exchanges and made bounty. But their budget is too low and there are so many participants, eventually each person only receives 2-3 $ for 1 month of work