Great question! I have wondered the same thing.

My guess would be the US Govt would argue that it is not "benefitting" from the cooperation:

"Involuntary servitude or Involuntary slavery is a United States legal and constitutional term for a person laboring against that person's will to benefit another, under some form of coercion other than the worker's financial needs." [1]

Or..that Apple is not a person. Or..that Apple is not being coerced.

Who knows :/

1. Wikipedia: https://en.wikipedia.org/wiki/Involuntary_servitude