I will get charged whether I use it or not

my contract ends on 11/12/2017. I gave notice yesterday for them to cancel it the day the contract ends , giving enough time in case there is drama! so i get a call today saying whether they cancel my contract on 11/12/2017 or 31/12/2017, i will be charged the full amount! so iam saying it is unfair, because when they activated my contract on 11/12/2015, I was charged pro-rata, i only received data for the days it was activated and only paid for that, so why can they not do that same when i cancel???? I hardly have signal, and half the time I can not use my phone, i would like to know why they charged pro-rata when it was activated, and can not do the same now when I want to cancel???