In some ideal world, where both parties of an exchange hold equal bargaining power, the "free" market might be "voluntary".
This is certainly not the current state of affairs, though. Being able to "choose" between a crappy option and a horrible option is anything but voluntary: "you can choose to work for me for little to no money... or you can choose to starve to death. it's your 'free' choice."
The very premise of what would make a free market efficient in the real world simply doesn't exist, but we keep trying to convince ourselves that it does because we don't know or aren't used to anything different.