I read another article that was much more in-depth. I believe that some phones are sold unlocked even if the carrier has the option to lock it (iPhone 5 on Verizon was cited as an example).
Also, I believe this only applies to phones sold at the insanely discounted prices when you sign a contract. So if you were to go in and pay full retail price without signing a new contract, your phone is unlocked.
Additionally, contrary to what was posted by Mowgli, the other thing I read said that after the contract is up they will unlock it for you, though you still can't do it yourself.
Again, this was all just what I read through another source.
When the company is so heavily subsidizing your price, I can understand them not wanting to you unlock the phone to access a bunch of shit they'd rather monetize. Pay full retail, get what you want, simple as that.