[Hermione] explained the Society for the Promotion of Elfish Welfare to a bemused Ron and Harry, saying that when they joined, the society will have three members. Ron protested that the House-elves are happy as they are, but eventually joins.
Hermione managed to get several students, such as Neville Longbottom, to join (paying a fee of two Sickles), though they only did so to stop her from badgering them. These students included a reluctant Ron Weasley (who thought S.P.E.W was a joke because its name reminded him of gagging) and an indifferent Harry Potter.
She also offered it to Hagrid, but he refused as well, saying that the elves liked to work. Fred and George Weasley also put in that the house elves were happy to work at Hogwarts.
Becoming infuriated with Hermione's "obsession" with the Society, Ron Weasley started calling the group "spew" and, on occasion parodied the name by inventing S.P.U.G., "Society for the Protection of Ugly Goblins". Hermione angrily replied to this by pointing out that goblins, unlike house-elves, were capable of defending themselves against wizards on their own.
Around 28 June 2011, when a Hogwarts student noticed a S.P.E.W. badge amidst Winky's stuff, Winky explained the organisation to them and how house-elves should be ashamed of it.
Taken from the wiki. The house elves were seen as happy to be slaves, and any attempt to free them was both unnatural and misguided. If you're looking for a more in-depth review of why the Harry Potter series was problematic, Shaun made a video about it about a year ago, and he actually (re)read the books before talking about it, unlike you or I. The section on slavery begins at 37:47.