When I was in high school, the cheerleaders (and football players, runners, etc) had to wear their uniforms every game or meet day. In fact, our version of the glee club wore our ‘casual’ uniforms every day we had a gig (and we were kind of a big deal, for a high school show choir, so we were in those t-shirts a LOT). It would make sense for Sue Sylvester to require the Cheerios to be even more visible, as they’re international champions and being on the squad carries with it a huge pocketful of pride. Adds another level to Quinn’s symbolic de-uniforming. Sort of like when Mr. Dawes Jr. punches through Mr. Banks’ hat at the end of Mary Poppins.

Just a guess though!