There is a major problem with these bounties, and it is that in order for the bounty to be paid out the person needs to be convicted.

In most developed countries, there needs to be a clear chain of evidence in order to secure a conviction, essentially, law enforcement need to prove that all of the evidence was obtained legally and through the proper channels. It's quite likely any evidence obtained by a bounty hunter would be inadmissable as it would be difficult for LE to prove all of it was obtained legally, and for that reason its highly unlikely prosecutors would move ahead on a case like this.

See:
https://en.wikipedia.org/wiki/Fruit_of_the_poisonous_tree

It would be much better if the bounty was paid for providing the identity of the person along with proof that is acceptable enough for the escrower.