In the US, carriers lock down their phones to sell contracts - thats their business motives. Sadly all the national carriers use differing network technologies making it difficult to sell phones unlocked and their high prices makes subsidization the only viable business model. Carriers exert huge amounts of control regarding handsets to entice long term contracts. People just don't "switch carriers" all that frequently.
Thanks for that answer. I wasn't actually talking about contracts, but about exclusivity. With the 3G, I opted for a 2 year contract to subsidise the cost of the phone too, but I had a choice of providers. There are at least 5 carriers all offering competing plans for the iPhone 4, so we're quite spoilt for choice.
You hear a lot of complaints from international customers about paying higher prices than in the US, but I'd take our system any day over having no choice of carrier.