I got the impression (I don't know why) that hags and warlocks were some sort of in between class between squibs and witches/wizards. Maybe an uneducated variety of magical person. The reference I remember was that they lived out in the country somewhere, somewhat outside of the wizarding community, so maybe that's why I'm thinking that. I'm picturing Billy Bob out in a cave somewhere mixing up a random potion at this point.