Even if a reasonable expectation that no-one will be harmed during evacuation exists, it doesn't follow that forcing an unnecessary evacuation has no expected costs other than inconvenience (as you originally claimed).

It might be, for example, that most experts in the field accept that theatre-evacuation technology simply isn't expected to reduce the expected cost of an evacuation below .01 deaths, and thus expecting less than that is unreasonable, and this fact is published and known to all theatre-goers, who decide that .01 * likelihood-of-evacuation is small enough to not preclude going to the theatre.

It follows that there's no grounds to sue the theatre, the fire marshals, etc.

It doesn't follow that forcing an unnecessary evacuation has no expected cost.