This is a question that's going to get answers depending on the culture of the people responding to it, no? If I'm not mistaken, it's far more acceptable for young adults to live with their parents rent-free in Asian countries than it is in the Americas (for example).
The way I see it, a parent brought a child to this world; the kid had no say on the matter, only the parents do, and they proceeded to bring the kid to this hard, harsh, horrible world where he/she has to work every day for the rest of their life in other to survive in this world that they didn't ask to come, the least the parent can do is make everything about that child their reponsibiy. So until that child is old enough to go live on their own and take care of themselves, the parents have to take that responsibility without complaining because if they didn't want the responsibility, they simply shouldn't have gotten a baby in the first place.
It is the responsibility of the parent to teach the kid to be considerate and responsible and also be able to take care of himself so that when he is of age, he would be fine on his own, but if he somehow still stays with his parents, he should can desice to help out with the bills (if he's working) because he has been trained to be responsible and considerate. not because it's his responsibility.
I know it may sound extreme, but it's not. It's just something parents fo not consider before getting a child.
A lot of parents feel children should be grateful to them for bringing them into this world, but it should be the other way round. The parents should be grateful to the kid for coming into their lives because they were the ones who wanted a child, and they got what they wanted.