I have been there and know is sounds illogical. You do know that the "replacement" phone is not actually coming from T-Mobile correct and it is a 3rd party company. (Assurant)
My guess is the vendor that gets the JUMP trade ins will only take working devices so that T-Mobile gets the money to essentially pay off your EIP balance and eat the difference between the two which ironically if I understand it correctly is Assurant also but a "different" part. I am sure if it were all in-house it would a lot easier to cut out some of the nonsense.