Compatibilism rests upon the concept of philosophical zombies. It says that since you can't know someone else's internal state, you can assume they have free will, because assuming they don't gave free will doesn't change anything you can observe. It basically says that free will is an unanswerable question, so it swaps in a different question that is answerable.

Its akin to p-values in statistics (probability of evidence given hypothesis) being used in place of a what we really want to know (probability of a gypothesis given evidence).