"Some people would be happier as slaves." is an is-statement -- it's either right or wrong (true or false) as a matter of fact, regardless of morality.

I agree generally with your point, but this sentence assumes "happier" is an objective quality - which may not be true. if we were to taboo "happier" in that sentence, the new phrasing might include a moral claim. Consider:

"Everyone is happier if jocks can haze nerds without complaint" --> "Jocks by show virtue by hazing nerds, and nerds show virtue by accepting hazing without complaint."

The second sentence contains a number of explicit and implicit moral claims. Those moral claims are also present in the first sentence, just concealed by the applause light word "happy."