Cancellation policy surely not legal

I have a 24 month iPad contract which comes to an end on 3 May 2016. I phoned the call centre today to make sure that the contract is cancelled on that date and not rolled over. I was informed that the policy is to process all cancellations on the last day of the relevant month. Therefore even though my contract ends on 3 May, I will need to pay for service for the entire month of May. Surely this cannot be legal? I did not contract for service beyond 3 May, I do not need it and I did not ask for it. How can it be a legitimate practice for an internal company policy to override the legal contract in place?