There's https://en.wikipedia.org/wiki/2010_Austin_suicide_attack but I still think it's pretty rare.

Somehow this didn't show up on my radar back then

I remember the day it happened, a lot of people were suggesting that Glenn Beck's far right rhetoric encouraged the person to do what they did, so Glenn Beck spent his entire 5PM tv block saying that he had nothing to do with far right terrorism.

If that's supposed to be a joke, it's in poor taste.

As many of the best are.

Why is it that I would need to prove that?

Do the laws of physics somehow not apply to nuclear reactors and planes until someone has crashed into one?