As far as I know, all the cellphone companies do the same thing. They sell an initial contract which includes a charge towards the cost of the hardware supplied. If, at the end of the contract period you don't upgrade and renew the contract you continue to get billed at the old price. You are, therefore, still making a contribution towards the cost of the hardware even though it has actually been paid off. They do not automatically reduce the monthly charge by the amount contributed to the cost of the hardware.
Vodacom has a new twist to this. If you've been receiving a discount on a renewed contract because you did not want new hardware they DO increase the monthly charge automatically at the end of the 24 months.
Talk about double standards!