Herminny, house elf treatment depended on the household. Can you imagine the Weasley's mistreating their elf? But if was in the book, the Hogwarts elves were treated with respect. There is a difference between respect and slavery. A slave can be treated with respect, but it's still a slave. House elves might not have rights, but that didn't mean their owners explifically treated them cruelly.

It is slavery love. They're owned by the family, and often passed down through generation. They must, yes, do what their master tells them, but not all families treat their elves like scum. Your making an assumption on the little we've red about these creatures, it's impossible to make a statement such as 'All magical families treated their elves cruelly' as we haven't enough evidence. But if JK was trying to portray these creatures as a kind of view on other races then it's fair to say it was a pretty equal balance. There are some countries in which other races are prejudiced and bigoted, but others in which they are treated like all other citizens. Same with the elves.