Setting off a fire alarm would indeed be just as likely to set off a panic scramble for safety, at least in principle.

Though in fact in my experience when an alarm goes off in most public places, such as a shopping mall, people seem to stroll around assuming it's just that it's just something wrong with the alarm. I imagine if someone shouted "fire!" The assumption would be it was some twit larking about.

I'mm surprised no one has mentioned the Twitter Joke Trial in England when a young man called Paul Chambers, after been inconvenienced by an airport being closed, posted a message saying "Crap! Robin Hood airport is closed. You've got a week and a bit to get your shit together otherwise I'm blowing the airport sky high!!"

He was charged with sending a "public electronic message that was grossly offensive or of an indecent, obscene or menacing character contrary to the Communications Act 2003", and fined £385 with £600 court costs, and also lost his job. After widespread condemnation of the case in the UK media, and a series of appeals, the conviction was overturned and the court ruled that "a message which does not create fear or apprehension in those to whom it is communicated, or who may reasonably be expected to see it, falls outside this provision [of the 2003 Act]"

Which in the end seemed a sensible outcome, and might have been an appropriate opinion for the court to take into account in this American case. Allowing for a different understanding of irony in the USA, perhaps.