A consveratory is not what you would call a classic indoor environment, it is after all meant to emulate an outdoor environment indoors. Yes it is indoors but that's being far too lenient in exactly what indoor environment means. Same thing with the cell phone refrence, sure cellphones go outdoors but they're also usually shielded from direct contact with the environment, either in a bag, or a holster that is not typically in direct contact with sun for excessive amounts of time.
Those symbols are meant to be descriptive without requireing language, because a tiny icon would describe more than the 4 pages of text that would have to be in it's place to explain what it means it ever language.