Helen wrote about the Federation administration:
They fear political crimes so much they would rather covertly slaughter large groups than try to brainwash them.
And Jenny responded >Why spend money brainwashing these people? It's only the leaders that need brainwashing and that's only so the Federation can use them as political pawns.
When a government would spend money on a surgical implant on a killer, and pay the high cost of transportation to send convicted criminals to planets like Cygnus Alpha, one assumes there is a high need for labor that makes it worthwhile for the government to pay for these things. I would be surprised if brainwashing is very much more expensive than convict transport. However, it could be considered more reliable, as dead men do not throw off their shackles.