Also, the Prime Directive would seem to apply here. These colonists renounced their Federation citizenship and lived as citizens of the Cardassian Union. So if the Federation tried to interfere, that would be a breach of the PD and could lead to war.
Indeed, wasn't Eddington bragging about how the Maquis were going to declare independence? (Although that could have been an instance of Eddington being the egotistical jackass that he was.)