Because it is completely reasonable for providers to give you a locked phone for a price subsidized by a 2 or so year contract. Nothing stops you from buying an unsubsidized, unlocked phone.
That's how things work in Greece. Carriers have no business messing with phones in any way (putting apps on them, locking them, etc), and they don't. Every single phone in Greece, no matter how long the contract, is stock unlocked from the provider.
Outlawing locking phones would drive phone prices up, I'd think.
So have an early termination fee that constitutes the amount of the subsidy on the phone.
One use case for unlock-at-will (or no lock) proponents: The user is in the U.S. and has a GSM phone. The user travels to Europe. Roaming rates on that GSM phone are very expensive. If the phone is unlocked, the user can purchase a European SIM and take advantage of local, European rates (probably using a "preloaded"/"reloadable" cash-balance SIM).
The user is still paying his US$60/month or whatever on his home contract. S/he is merely avoiding high roaming rates while traveling, without having to purchase a "disposable" phone that 1) Probably has less capable physical specifications and does not have any of the user's data; 2) Is more likely to be discarded and become a disposal problem / waste at the end of the trip.
Unlocking is not just or primarily about post-contract use, nor is it about escaping a contract without compensating the provider for the unpaid portion of their subsidy. It's about being able to use the phone the way you want, once and as the obligations one has committed to have been met.
And... it potentially helps the waste stream / disposal problem, by allowing the transfer of older devices to other parties who may be happy to use them, effectively extending their useful lifetime while avoiding now unnecessary production of additional units.