There are definitely places in a theater, out of the view of spectators, but likely fire hazards, e.g. the projection room.
Most building fires cannot be seen until it is too late. Smoke is an early indicator of a fire, but it cannot be seen in a pitch dark room.
And even in perfect conditions to determine if there is a fire or not, the audience may not be capable to judge if there is one or not, e.g. children.
I should be immortal, all knowing and beautiful but I'll have to make do with what nature gave me.
It can very well be a principle of law. In certain legal codes, it actually is.
As for reality, that's a pretty low bar.
We have many "shoulds" (e.g. you should not cross a red light lest you get a ticket / prison) that are not some inherent property of reality or physical law, but things we've decided upon.
In fact, those are the only things that makes sense to consider as "shoulds". The constraints of reality are not negotiable, so should there is superfluous.
And people should not be idiots to be panicking from someone yelling "fire" when they can't see one in the theater...