That's pretty much exactly right. The law is set up to protect minors from being exploited, because they are presumably dumber and easy to manipulate by adults. If both parties could void the contract it would defeat the underlying reason for the law (to protect minors). That's how I understand it from what I remember of my business law class in highschool like 10 years ago. So, while it gives minors the possible ability to screw adults, it is assumed that adults are much better equipped already to avoid being manipulated by an unethical minor and don't need special protection. I would tend to agree.
edit: I might add that minors tend to be rather innocent, while adults, in general, are evil, ungrateful ---uvulas---.