The best rationale for subsidized/lock is...
a) To effectively have devices appear 'less' expensive, and promote high turnover of devices
b) Not allow consumers to 'buy'/'resell' new devices that were subsidized. Eg. I have a +4 year old iPhone 3GS, I can obtain a new or refurbed iPhone 4/4s/5 that is highly subsidized, and sell it for ~$300/400/500 and make a little profit. Carriers would like to keep those devices on 'their' network regardless, as they 'subsidized' the device.
c) Aftermarket vendors love the US subsidy style (vs. BYOD) because many users have been 'trained' that devices are to be replaced after 2 years