The theory and phenomenon has actually very little to do with “pranksters” or with any sort of crying wolf syndrome. Studies have indicated and shown repeatedly that when a victim screams for help, the people that can hear the victim do not respond. On interviewing the people they all remark that they did not want to get involved, were afraid of retaliation (even when their identitiy would have been anonymous by simply calling 911), and “thought someone else would do it.” This is called the notion of Diffused Responsibility or the bystander effect. The most famous case being Catherine Genovese who was killed in her apartment complex in 1964 while screaming, “he stabbed me, he stabbed me.” While largely considered to be inaccurate reporting, the bystander effect still stands. Another example involves a woman being raped on the street of New York multiple times while people walked past. The Bystander Effect is also thought to be seen in the creation of the Holocaust, the torture and murder of Ilan Halimi, and the gang rape of a 15 year old girl on the floor of her Homecoming Dance at Richmond Highschool.
Screaming “Fire” or “Bomb” while not a foolproof way to do so, grabs the attention of another passerby to alert them to danger that might be happening to them. Merely someone running around the corner to ask “why the hell are you screaming fire when there’s no fire?” would be enough to stop a rape. If I had to choose between being raped and someone being angry at me for calling fire when there wasn’t one, well someone else is getting their panties in a bunch. Not me.
Also, people might flee from Fire or Bomb but they will call the police or fire department. "Some crazy woman is screaming about a fire or bomb," will get them there pretty quick.