The house elves are content with doing their duties despite being treated "immorally." Dobby was the only elf who supported Hermione's S.P.E.W. Cause and his efforts were halfhearted at most. The rest of the house elves considered it insulting that someone wanted to relieve them of their duties and "free" them. Advocating for the liberation of house elves would only cause conflict with the house elves themselves.
And that's really what we're talking about here: a group of people, the house elf liberators, trying to force their understanding of right and wrong onto others, harming them in the process. When you try to avoid adding personal beliefs to SPEW, one is reminded eerily of the concept of "The White Man's Burden."
The goal of society should be to maximize the happiness of all it's members, not to fulfil one vocal minority's values.