It's not the offense, it's the wasted time and money. Think of one of those meeting timers that counts in dollars instead of minutes. Now apply that to all the time spent by random people calling the main office, and by the main office fielding all those calls. It's one thing to cost your employer thousands of dollars because you made a mistake (I'm sure we've all been there), and quite another to cost your employer thousands of dollars with a prank.

You can't even make the (quite bad) defense that people should have known better and it's their own fault for falling for it. The message was 100% plausible.