...people don't live only on Earth anymore. Thanks to technological advancement they colonized whole solar system. Mankind now habitats planets, moons, asteroids and countless space stations throughtout entire solar system.
In world like this, criminals can easily hide anywhere. That is the right place for bounty houters. They help police to clean the society from dangerous people. Suddenly the world seems to be somehow similar to times when people colonized "new world" on their own planet centuries ago.