In this particular case, I think the point is less 1 or 2 but more point 3

(3) the contrapositive, where you continued the flight, it really was someone stupid enough to name the broadcast name of a bomb "BOMB", it goes off, and now you have to explain to the press "we thought nobody would be stupid enough to really name it 'BOMB'"

So you assume it's a low risk event, and tell everyone onboard to turn off their devices to remove the chance it's just someone making a bad joke or a coincidence, and then you end up with the outcome of trying to avoid having to say that in a press conference where everyone is already primed to think you didn't do enough.