Insurance for the promoter of the show, they check extensively for hidden nitrous systems.
Basically carbs, tunnel ram or single 4bbl, and rev the piss out of them.
No, you didn't understand the question. How is a blower truck any more dangerous in that situation than a truck with a pair of Dominators on a tunnel ram, feeding a 600+ci engine?
And more to the point...why don't they use cutoffs, again? I remember those on pro mud racers in the early 90s!