It wouldn't be "forcing" with threat of imprisonment and Apple given the power of police, judge, jury and imprisoner such as a government has; it could be written into the terms of the contract - "we will buy from you, if your product is up to our specifications, and its manufacture is up to our specifications, and that the manufacture is done on ethical terms described in clause B, and the contract depends upon us being able to send auditors to all premises involved in our component supply chain and talk to all employees so involved at any time".
If there is no company willing to agree to a contract like that, Apple would only be able to force them by offering more money until it was worth their while.
It's no more /force/ than employers have over employees - do what we say or lose your income. Which is not exactly a level of force I like all the time, but if you agree with contracts and employment in general then adding "and treat your workers well and let us see it" doesn't seem like a hill to take a stand of fundamental opposition on.