It really depends on the whole situation. A parent won't demand a rent if they are not financially struggling themselves. They won't ask something for their children as long as they can bear what they are going through.
Plainly wrong assumption from your part.
My parents weren't wealthy but more or less well off.
All I know is that no matter how wrong a thing is wrong, you will still see some people defending it, which is why I don't let anything surprise me in this world anymore. I would rather leave that house than for my parents bill me. It's a different thing to be supportive as their child or urge you to be suportive rather than making it mandatory, that's obligatory, it's off. I don't see that as a family anymore, so what? It's not worth it.