Mortgages, broadband, television, utility bills and just about every type of recurring payment has some provision for what happens when you complete your fixed term and not a single one of them puts the price down automatically at the end of it despite you having paid off the cost of your Sky box or router.
In half of those cases there's no customer equipment to take account of, and the majority of the remainder its chickenfeed (like a fifteen quid router). Now consider that the majority of these deals have a big discount on the contract for new customers, and you'd realistically expect the cost to go up.
For PVR there's more at stake, I'll grant you, but the case of mobile phones is totally different. The network operators are going out of their way to disguise the handset hire-purchase element.