True enough. And there is an ancient history of slave soldiers, particularly in the Islamic world.
But a slave, by definition, is no more a volunteer than a horse is.
This is, of course, why slavery is very nearly the ultimate evil of which humans are capable.
It is true that most of the black soldiers in the Confederacy were slaves who were fighting with their masters. However, there were many free blacks living in the South at the start of the Civil war, some of whom voluntarily enlisted in the Confederate armed forces. To deny that this well-documented, albeit minor, fact occured is historical revisionism and political correctness run amok.