When you purchase your device with service from AmazonWireless.com, we automatically pass along an instant discount from the carrier to you. This discount has been provided to you based on your agreement to (a) activate a new, or extend an existing, line of service for this device with the carrier, and (b) maintain this service in good standing for a minimum of 181 consecutive days. If you do not activate or extend a line of service in connection with this device, or if your service is canceled/disconnected before 181 consecutive days, AmazonWireless.com will charge you $250 per device, plus applicable taxes.
Amazon adds an ETF, and provides a bit of insight into how "authorized resellers" make their money. Interesting that the whole ETF expires at only 6 months into the contract.