I can see where you are coming from. How can we forbid clothing if the goal is to not dictate what to wear?
But consider that in a community, be that at school or in the neighborhood, classmates and neighbors can uphold unregulated, religious rules. Is it free choice of clothing if the law doesn't forbid anything, but only girls with (insert appropriate clothing) are allowed to join in the play? And there is plenty precedent of religion that causes precisely such group behavior.