The legalistic definition pretty much is the definition used by philosophers who are physicalists (such as Dennett). Basically, the question is not whether physical processes determine what you do; of course they do. The question is, which physical processes? The ones that are going on in your brain, that we label with words like "perception", "cognition", "decision", etc? Or other processes, like the process of someone holding a gun to your head?

The only real difference I can see between the legalistic definition and the philosopher's definition is that the legalistic definition of "coercion" is broader. For example, I mentioned someone holding a gun to your head just now. Legally this would be coercion and wouldn't count as a free decision on your part. But to a physicalist philosopher, this might still count as a case of free will on your part--just free will based on a highly constrained set of acceptable choices given the circumstances. But that's a fine distinction that doesn't change the basic point.