Perhaps. I think the line is drawn when you don't have a choice as to the nature of the work. Which raises the thought-provoking question of how little choice is too little - freedom to choose which section of field you pick cotton from is obviously at one end of the scale. What's at the other? What about "you can work at any job you want, as long as you never leave the country or speak ill of Glorious Leader?" How many choices do you need to be offered before it doesn't "count" anymore?

Agriculture itself must have felt like slavery to hunter-gatherers - work the fields, or get killed by those who do.