The pirates from History books attacked villages in the coast to catch slaves. Should we ban Peter Pan and all books for children with pirates?
There is slavery in DS because it is a postapocaliptic dystopia. If there are slaveres is becaue these are to be kicked-ass by the heroes. The metaplot starts with the death of Kalak. This means the sorcerer-kings aren't totally invincible and they can fall.
If there is a reason because I wouldn't feel confortable with the slavery in the fiction is the idea this can't be avoided. We have to stop any possible "
learned helplessness". The fight against the slavery can be very slow, hard and painful, but the end of this is not only possible, but also totally necessary.
Maybe the sorcerer-kings with their psionic powers mind-controlled the population, at least for the first generations, but after a century it wasn't so necessary. One of the reasons to allow enough free will is for the generational replacement and to avoid demographic crisis the citizens should can fall in love and create families.
Other idea those tyrants should have learnt is "being loved and feared is better than only feared", "the cruel rulers can be betrayed (or suffer rebelion) more likely, unnecessary cruelty must be avoided" and "the positive stimuli are stronger than the punishments, that is why gamblers do not punish with the lost money".
And in the Athasian Tablelands they couldn't buy more slaves from a far zone. They had to worry about taking good care of their "merchandise" because if this was broken by the abuse they couldn't buy a new one.
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A DS videogame is possible, for example about building a camp for futigive slaves.
The movie "lethal weapon 4" was about slavery.