I have hinted on it before: What the law says might not be the final word, the airline is free to make its own "house rules" because they run the plane and the airport has some discretion in who (and what) they allow in their buildings. There is a fundamental difference between government rules (the law) and airport/airline rules: when you break the law, the government may take criminal action against you; when you break airline or airport rules, that is a civil affair between you and the airport or airline.
I just want to add one point: airlines and airports can not just make up rules on the spot; passengers have to be properly informed of rules (and rule changes) as the rules for carry-on luggage are technically part of the conditions of your contract for carriage.