There are two ways of looking at this, but only one way that Disney does. First of all, Pocahontas is the daughter of a Chieftain, so while not technically referred to as a "Princess" by her own people, she is certianly analagous and the European colonists would have used that term for her. In fact, more questionable is Mulan, who was not a princess no married into royalty. Of course, they are both Disney Princesses.
When you look at this from Disney's view, "Dinsey Princess" is not so much a rank or title, but actually a brand. Many actual princesses are not "Dinsey Princesses" at all, like Kida from Atlantis, Vanelope Von Schweetz, and even Anna and Elsa. Elsa is actually a Queen, however neither of them are in the "Disney Princess" group. So, in teh end, it really has nothing to do with their actual status in their world's nobility, but really jsut the label Disney wants to place on them.