I consider myself quite well versed in crazy bombings and shootings in American history. I have watched probably hundreds of hours of footage from 9/11, Waco, Ruby Ridge, Columbine etc.
But never once have I heard about this bombing, and never once in my 26 years on this earth have I ever imagined that a state fire marshall would be willing to stand next to a live bomb to examine it.
The IRA also phoned about many of their bombs. The Omagh bombing is mostly notable because their phone-call didn't accurately identify where the bomb would detonate, and so got people killed.
Plenty of terrorist groups have recognized that killing innocent bystanders isn't good PR.
The IRA usually (plenty of exceptions and changes over time) phoned ahead, they wanted the impact of terrorism without the bad press of civilian killings. Not a particular relevant example in modern times though.
I wonder if it's more about phsycology, someone making a bomb threat is seeking attention for some reason and ignoring it might make their behavior escalate.
The Weather Underground planned one of its bombings to take place at night to minimise casualties, although they ended up killing some poor grad student who was working late.
That's not limited to any one bombing -- it was pretty typical of them to time their bombs would go off at night, as well as phone them in. They would also locate the bombs in places where they were relatively unlikely to hurt people: in the floor drain of a restroom, in a disused room, up in the ceiling.
There's a fascinating history of the militant revolutionary movements that cropped up around 60s, Days of Rage by Bryan Burrough.