What is the justification for the rule that,generally speaking,a minor can disaffirm contracts for necessaries,if a parent or guardian supplies the minor with such necessaries?

The whole point behind the rules regarding necessaries is to ensure that minors can acquire the basic necessities of life.If minors could disaffirm contracts for necessaries,adults might lack an incentive to deal with them regarding necessaries,which might mean that minors would not acquire them.Where a parent or guardian already supplies the minor with necessaries,however,there is no need to protect the minor in this way.Here,therefore,the minor can disaffirm.