It isn't when they've actually told you as much.I believe that this is all just speculation. If you see that the person is wanted red lettering, then only after that you can claim it.
Regardless, it's the decision of the player. In an anarchy system everyone has red lettering anyway (lawless) so it's up to the players in that system. Outside that, you get a bounty and notoriety, so you lose that ship for BGS work for your faction for a couple of hours.
Equally players in powerplay could pledge to your faction to 5C it. Nothing you could do about those either. Neither system is perfect and in the end it's player preference.