It sounds like you say that everyone should be able to wear whatever they want at graduation. But first you ask why it should be that way.
Here is the problem with not enforcing the sentiment of the people making the rules: The people making the rules are paid by taxpayers to run a public high school, including graduation ceremonies. Open the dress code to any and all expression and you will have people wearing offensive things like Nazi emblems, Racist slogans, obscene words, and pornographic pictures. The taxpayers will complain and they will be right to.