I've had an experience where they claimed a cable modem I had purchased from Amazon was their own (from the MAC address on the label). I had to call in several times because I had loaned the modem out to a friend and they refused to activate it because it was an "unreturned rental modem". They're not even able to keep track of their own inventory, I would hardly count on them keeping track of MACs of all modems for the purposes of data caps.