Broadly, the FCC is mandated to: >make available so far as possible, to all the people of the United States, without discrimination on the basis of race, color, religion, national origin, or sex, rapid, efficient, nationwide, and world-wide wire and radio communication services with adequate facilities at reasonable charges.
But I don't see anything in there that say they have the ability to mandate unlocking. You can argue that it falls under the "adequate facilities at reasonable charges" portion.
I certainly want the ability to unlock my device but is the FCC mandate legal or not?