> Slavery is immoral because it takes away freedom of movement . . .

Here's a counter-example: the difference between a freeman farmer and a slave farmer is not that they don't have freedome of movement, but that the freeman has the right to sell the harvest to whom he wants while the slave does not.

It sounds like the definition of slavery as lack of freedom of movement was dreamed up by some law professor in order to avoid having to characterize the income tax as a form of slavery.