Slaves are property in captivity compelled by force to work on whatever their owners choose - there’s nothing to tax. Citizens are free to not work or move away, but if you choose to stay, you gotta pay the “protection racket” tax. It sucks, but it ain’t in the same ballpark as slavery.
Discussion
You think slaves will be assigned to do things they aren't good at? Working for money, then the government using your money for whatever they want, is in effect no different than being forced to perform some specific task.