if there are no bounties out, they'll still attack, supposedly going for trade ships when possible. That makes them pirates. In game, however, most people treat them as somewhere between mercs and privateers. Essentially, though, they remain pirates. They attack targets with a high bounty in the hopes bf being paid for what they would normally do anyway. If they were mercenaries, you could hire them and not worry as much about them being bought out from under you, and you could direct them vaguely as to what you want done. Remember that pirates and bounty are two separate but interrelated systems. Pirates will try to take advantage of bounties the same as anyone else could, but the bounties are not there just to buy the allegiance of the pirate faction.