If you're unable to restrain from acting on your passions, if your instincts have greater power than your restraint - or your ability to be indifferent to the lure of abusing your instincts - then you're not free. They're in control rather than your mind and heart.
"Freedom of restraint" refers to this. Maybe more accurate to say "freedom of being able to choose restraint."
The question is at which point do our passions become 'abusive' and require self-restraint.