There's a large difference between having some of the money from your wages (which can come from absolutely anything you can make money on) garnished by the state and the state telling you what you have to work on.

But the modern state can draft you if they decide to, so the state can make you a full slave if it wishes to. Also with the amount of regulation in the U.S. they can deny you from working if they decide not to give you a business license or one of the many other licenses you need to work legally(See this video for more info). So if you piss off the wrong bureaucrat you aren't allowed to do what you want to for work. Also the only difference I see from taxation and slavery is the amount taken. Slaves just have 100% of their income taken instead of 20%. Is it better? I don't know, but in both circumstances someone else is imposing their will on you and taking your labor by force. So if a slave master let his slaves become 80% more free than they were, that is nice, but they are still slaves. If I really owned my body 100% then I would also own the work my body produces 100%. Because with out the fruits of labor life isn't even livable.