Anyone will treat any person who's different either physically and/or mentally as less human, no matter what.
I think one of reasons why people were encouraged to make deaf take oral method is that it bring a sense of "normal" to deaf person. Like if they could speak and understand another person, then they are normal.
But I am sure there's many different reasons for that. Some parents don't want to learn sign language, some think that sign language would harm deaf children, that oral methods are the only method available in their area, that it's easier for deaf people to communicate with hearing people, etc.
So one of your suggestion is one of many possible reasons for raising deaf orally.
I can't hear and only communicate through sign language. But I never really felt like I was treated badly. Just that each of my action "surprise" them. Like they were impressed when I joined sports at public school or actually went to college. Like it's the most amazing thing ever. It's ignorance and assurance that did the most harm.
