"Maybe I'm being naive, but where is the problem with this arrangement? "
What you are talking about -- phone subsidies, is primarily, dealt with via "early cancellation penalties". Carriers also check your credit before "advancing you" the cost of a phone to verify that you are an acceptable credit risk.
Phone locking allows companies like Verizon to lock out features of the phone. Example: not being able to transfer [music] files from my computer to the device.
My phone had the capability to transfer music files over USB, but Verizon locked out this ability, to encourage me to use "air time" and "data minutes" to download my own music to the device as well as paying per-song charges at the time.
Then comes the issue of being able to take my phone with me -- AFTER any contractual-obligation period, to a new carrier. This was (and with lock-in, still is) doesn't allow me to use a phone I've, *long since*, paid for.
Phone locking has little to nothing to do with something that is already dealt with via early cancellation penalties and Obama didn't ask that early cancellation penalties be abolished.
Phone locking disallows consumers bringing their own device to a network (presuming the device is network compatible) and is used to artificially inflate the costs of services and features long after any contractual-obligation period.