You are very wrong,
If someone under the age of 18 signs a legally binding contract and disobeys the contract, the parents (or legal guardians) are held responsible and the law treats it as if the parents signed the contract, all of you kids out there who think you are pulling a fast one on this site and the law, note that your parents could be sued for any of your mistakes.
Let's say some kid decides he is going to scam someone on ebay, so he goes on there, makes a new account (which signs a contract saying you cannot scam anyone), and scams some innocent person. The parents of the minor can be sued for their child's actions, and can be punished for not watching what their child is doing.