Funnily enough, they sell phones in the same way. Most phone unlocking is done by a guy at a street stall, for £10, and most people probably assume it isn't quite legal. The carriers aren't stopping subsidies pf phones, because they know if they don't the other carriers will.
What they do dictate however is the features that go into phones. How would you like a wifi enabled phone, that connected to your home LAN and made VOIP calls when you were in range. Out of range, and you get charged for a normal mobile call. The technology is there, but no carrier is going to commit to a phone that will essentially lose them revenue. Guess who the mobile phone manufacturers biggest customers are...? The carriers. Screw them. You ought not to be defending another business model that is holding back innovation, and giving customers what they really want.