Because they also may or may not have been available in the boys room. I think that was initially the intention but, well, some teenage boys are going to be dumbasses with menstrual products.

EDIT:

As for the latest round of name calling, it seems the aspect of the Minnesota law that has conservatives most agitated is its language: the law states that pads and tampons must be available to “all menstruating students” and “in restrooms regularly used by students in grades 4 to 12.” A failed attempt to amend the bill to only name “female restrooms,” did not keep it from passing as is with bipartisan support.