Why do they harp on getting a haircut and having a specific hairstyle so much?
The reason is not to control you; it is about functionality and appearance.
Your hair needs to be able to fit under your hat and, most importantly, your helmet.
The appearance of having well-kept hair is a sign of discipline and self-care. You are expected to look professional at all times. A haircut is something simple to maintain. When you look professional and show you care, you get more opportunities opened for you.