How free is anyone, really? Tell the fat man that he's completely free to stop eating too much and lose all the excess weight. Tell the alcoholic that all he has to do is decide to quit drinking, because he's completely free to do so. Tell any of us that we're altogether free to live a sinless life wholly pleasing to God. How often is it gonna happen, no matter how much we'd like it to. I'm free to get a PhD in Physics and a job as a professor, but how likely is it to happen?
And lets make it a bit worse. Homeless Jerry wanders the streets downtown, sleeps in doorways, eats out of trashcans, and spends what little money he gets on coffee and cigarettes (he doesn't drink alcohol or take drugs as far as I know). Most of the time, if you notice him at all, it's because he's standing on a corner or leaned against a wall somewhere, shaking his fist, and shouting the same swear word over and over and over and over until the spell passes.
Jerry (and he is real, BTW) is a slave to whatever is wrong with his brain. But you and I aren't, are we? At least not to the extent that Jerry is. But there's the thing. There are physical and mental limits on what we can, and what we will, do. Best intentions are fine, and will power is great, but it is, as the saying goes, not in us to command success. Cop out? Maybe, but the next time one of us actually manages to live without sin, then I'll put more stock in the sort of "free will" that brings salvation. For the rest of us poor knuckleheads jammed into a life of sin by our own mental and physical limitations, the direct intervention of a Savior is necessary. Our free will isn't going to save us any more than Jerry's free will is gonna make him sane.