What is a cancelation fee for when a contract has ended.

I took out a tracking contract with netstar for 36 months. On the 36th month after their debit went off I phone them to cancel the contract since it has lapsed. I'm told I have to pay a cancelation fee. Why is that because I've done my part for the duration of the agreement, why am I being penalized for a contract that we agreed upon when it comes to an end? Somebody please 🙏 shed light for me on this.