Library dress code????????????

I wore a sleeveless top (a very standard looking top) and the security guard stopped me and said 'aise jaoge library?, yeh cut sleeves allowed Nahi hai'. I told him I have come here wearing sleeveless before also and have NEVER been stopped. There was a well known faculty there (I am not gonna drop names) who also just sternly told me to put on a jacket and plan my outfits accordingly. I don't understand how this is supposed to matter. I understand the no wearing shorts rule but sleeveless ffs??