Here is part of the Act that gives the FDA power to mandate food safety:
>IV. Food
There is a distinction in food adulteration between those that are added and those that are naturally present. Substances that are added are held to a stricter "may render (it) injurious to health" standard, whereas substances that are naturally present need only be at a level that "does not ordinarily render it injurious to health
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?