Not sure that's true. Free will doesn't mean anyone can do anything. It means any decisions a person makes are truly decided by them, and they actually could have made a different decision.

People who don't believe in free will believe that the physical laws of the universe are deterministic. That leptons and quarks behave in ways determined by their state. That this is true even inside your brain, and thus decisions you make are actually just the result of particles interacting. Even quantum effects, though random, are not consciously decided and thus do not affect free will.

The circumstances you are in change the inputs to those equations, but they don't change the fact that the equations exist.