wondering why?

last night i saw this sign in front of this club inside of this quite prestigious hotel. this sign read : NO animals, weapon, sandals, shorts, outside beverages, rock t-shirt, allowed.
the list is understandable, except for the rock t-shirt thing. animals? a small poodle can tear the place upside down. no animals-agreed. weapon? who wants to dance in the middle of blood dripping from ones body? no weapon - agreed. sandals & shorts? well, maybe the management doesn't want their club turns into a beach party or jaksa's hubbub. no sandals & shorts - agreed also.
a rock t-shirt? what can a rock t-shirt possibly do to the clubbing society? can it cause anybody any harm?? no rock t-shirt - enigmatic.