Well if the ID is possible to buy, and so are the ships, someone is going to try to do it. Besides, isn't it possible that if the SCRA members got to Sirius, maybe other non-SCRA Coalition members got here too? Why shouldn't they get a Coalition ID? And once they have a Coalition ID, who's to say that they can't have Coalition ships?
I understand for the Phantoms and AW that they are exclusive factions that there can't possibly be independents of. Coalition members are likely scattered throughout Sirius, and they can't all possibly be members of the SCRA too.