The Magna Carta is being enforced upon him so that he follows the common law, the ones that all people follow. Even the peasants and serfs follow it!
Though the peasants are always working, they respect the law of the land, and in that case, are better than King John and the nobles.
Thank you for coming to visit me! I hope that my father learns what the kingdom is all about. Good day, traveler!
The common law is the laws that everyone follows, helping to create a better lifestyle. If one person does not follow it, then chaos can arise. That is what happened with King John, forcing the lords to create the Magna Carta.
King John was never a good king, but the Magna Carta had King Edward I create the first Model Parliament. But some Magna Carta laws are in rules of the ages now!