You bought from a 3rd party, I'm sure you accepted their free phone. Now you cancel on them, the shop doesn't or loses their commission because of you. And, they lose a telephone since you have it. Many cases, 3rd party authorized resellers have a store contract to not cancel in 6 months, else you owe them the cost of the hardware.
Example: Bought 1 Sidekick, 2 V3, 1 Nokia 6101 from a 3rd party T-Mobile reseller. I agreed not to cancel for 6 months, or else the store will take the retail value of the phones (at the time, $1,200) from my credit card. After 6 months, I can cancel with tmobile, but I will take a tmobile cancellation charge per line.