In the US, with Sprint, AT&T, & Verizon you pay the same price for service whether or not you bring your own phone, so you may as well take the subsidized device. To a person who buys a smartphone on subsidy for $200, as far as they're concerned that's the price and they expect an equivalent amount of value.
If the subsidy were out of the equation, and you had to pay $600 for that device, wouldn't you expect more from the phone than you would if it were perceived as a cheap throwaway? It seems like that would get the manufacturers to step up their game a bit, which is good for the consumer.