Flying an airplane does not require the signing of a contract that could be damaging to a company if rescinded later. Keep in mind that even if the minor's parent/guardian signs the contract by proxy, the contract can still be cancelled at a later date-- i.e. the minor could effectively decide later that the content submitted should effectively be 'unsubmitted'.

In the case of the plane, if the contract is rescinded, you don't fly.. that simple. For Apple, not so simple-- imagine what would happen if Apple were legally required to remove the changes in a critical file because a minor or minor's parents/guardian decided they no longer wanted to play ball.

The problem is that, just like a minor cannot sign a legally binding contract, once signed by proxy there are still special 'undo' clauses offered in the name of protecting the minor's rights.

(From reading all the noise on the subject, I have a feeling we don't have the full story.)