Hello,
I read the discussion. Disclaimer - I am long time friend of the topic starter.
It looks like he is alone here and probably another point of view is needed.
I have experience with iPhones (as well as TS) from the very first one, where we had to unlock them in order to use in Russia, where they were not sold till iPhone 3g (or was it 3?) as I remember. In another words - we have some experience using locked and unlocked iPhones.
Warranty is country specific - that is fine,since iPhone is working. The problem that TS is trying to elevate is that Apple has full control of the iPhones, particularly carrier/country restrictions, LTE capability. If you do a recover procedure, you can see iTunes contacting that central database.
When it comes to this purchase - I think it is chain of questions, where topic starter is trying get through:
1) this is unlocked iPhone. Price supports that statement, Apple probably (or must?) confirm that. It could be that locked iPhone was bought.
2) iPhone model (e.g. unlocked) could have been misplaced - they sold locked iPhone instead of unlocked. Target (the seller) should clarify that, but at the end - Apple should clarify that purchased iPhone was sim free.
3) UNLOCKED meaning. In the rest of the world (outside of US) this means only that you could use iPhone with any operator in the world. In US this could easily mean that you still could use with ANY operator, but ONLY IN US. That how marketing in US could be working, presuming US customers does not need anything, since they have everything in US. Apple could/must clarify that.
5) Now T mobile sim was used on the phone. Can sim lock iPhone? Yes it can, if manufacturer provided that functionality. Probably only Apple can clarify, T Mobile can also clarify. T mobile said they do not do that, prepaid cards probably should not lock the phones, because of the nature of prepaid cards use.
Apple has full control of iPhone and its functionality as I see.
I think in real life you never have to structure a problem into detailed puzzles with right questions and appropriate angles if there is a customer service, which solves all this for you, provided you are a legitimate customer (by using this I mean you paid the full price through legitimate trade channel). Apple probably can solve the mess of the different issues, which I described the above, piling up the chain. But Apple does not. That is the only question - why?