******* billing / Additional charges

I recently upgraded my contract to a Galaxy S5 on Smartplan 50 @ R349 per month. This was done on the 24th of June. I just received my invoice and everything was pro-rated correctly from 24-31 June, except the Device fee. This was charged for a full month's usage (1 June - 31 June). When I phoned the call centre they just told me that it doesn't matter if I take out the contract on the last day of the month, I will still be charged for a full months \usage"for the device fee. Nobody explained this to me when I did the upgrade at the store"