An electronic message costing a few pence - text, email, phone call or similar - shut down all LA schools. A similar event happened simultaneously in New York but was put down as a hoax.
Which city was right in its actions?
This time it was NY, but what if it was wrong; and what about the next one?
LA took it seriously and acted. It might do the same next time, but one day it will follow NY and be wrong.
This is how terrorism works.