Dude FRNs are counterfeit anyway.
You and I may recognize them as illegitimate, but the majority of people recognize them as legitimate. They're Federal Reserve Notes, and they're not claiming to be anything but Federal Reserve Notes -- therefore, not counterfeit.
What I mean is, if you stamp something that says "this bill is worthless" on what would otherwise generally be regarded as a valid bill with a given value, that's going to raise the eyebrows of a few clerks. Definitely not a good thing if, say, you just pumped gas into your car and you only have that twenty in your wallet -- especially if you're not even the one who stamped the damned thing, but you just got it as change somewhere else.