I'm looking for an older iPhone for my mom (5S or 6) and I'm checking Swappa which I've only used a few times. I see all sorts of different versions for the carriers and then different unlocked versions. If Swappa requires a phone to be paid off why are some of these still tied to carriers? Is it just that people are too lazy or don't know to unlock? I'd have thought even on the older iPhones they'd be either GSM or CDMA not tied to a specific carrier if it's paid off. I'm sure I'm missing something obvious.