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The Alternian empire keeps a large number of captive aliens as so-called "relievators" for its citizens to use. These serve an important function by giving the masses a safety valve for pent-up stress and frustration in an otherwise merciless and demanding society. Relievator use is physically pleasant and helps improve both mood, self confidence and general wellbeing. Everyone agrees on this.
There's absolutely nothing wrong with it. After all, they're just aliens, not actual people.
... Karkat would have been a lot happier if he could have seen it like that, too.