Bounty Hunters are a unique breed, earning their livelihood by taking out notorious criminals with bounties on their heads. Consider that some among their ranks were once on the wrong side of the law and are privy to the internal methods of the criminal underworld, giving them an advantage over law enforcement. In other words, they are not inclined to waste precious ammunition, let alone lives, if there's no payout in it for them. That's their purpose. Nothing extraordinarily complicated to understand.
Of course, some factions may deviate from this practice by taking out criminals without profit. However, this detracts from their purpose. There have been talks of a plugin being implemented that automatically awards BHs with monetary gain upon taking down hostiles. However, I have serious doubts that such a thing will be put into effect.
