Since January this year it has been illegal in the USA to unlock a mobile phone which is locked to a particular network, & which was bought on or after some day in January which I can't be arsed to check, unless they have the permission of the network operator. It does not matter whether they paid full price for the phone, or whether the contract on the phone has expired - nada. You are utterly dependent on the whim of the network operator. This is due to a regulation promulgated by the Librarian of Congress, under US copyright law.
Fortunately for Leftpondians, the network operators appreciate that it would be bad business to try to take too much advantage of the power they have been granted.