Because the cops then turn on you clearly.
Cops don't appear in HazRes and if you're that worried about your credits per hour that you cant lose one or two bounties you should be farming in a HazRes. Still it's honestly not even a big bounty shooting down another commander and it makes your point really clearly that you dont share bounties.
The system itself though isnt broken in anyway. It is no different than bounty hunting Irl eg. Someone else tracks the mark and beats him and while they are catching their breath you swoop in cuff the mark and haul him in, bail bonds pay you and I not the guy who did all the leg work. Is it morally wrong YES but that is the world we live in and the world used as a base for ED.