We can never repay our parents for the labor they did in our lives. I agree with you, they deserve our support in return but they shouldn't make it a law.
Some parents can be funny like you stated, they don't even train their children well but just keep putting high hopes pressuring the innocent young people to support them to the extend of making them do things that are unlawful just to make things right.
As parents, one needs to plan his or her life before retirement, you never can tell what life would look like tomorrow, your are children are not God, God forbid, some of them might not even be as big as you envisioned in life, so, why take chances? plan your retirement, plan your life so that you don't end up being a burden to your children someday.