Hmm. I have no doubt that you've seen drug dogs go bananas when they sniff money, and no doubt that you've seen the results of well conducted tests performed in a professionally run lab.
But it's essentially the same problem as testing metal for "radioactive content", then acting all shocked when every single piece of metal smelted since 1945 comes back with some degree of radioactivity.
Why 1945? Well, that's when atmospheric nuclear explosions started. Smelting consumes a LOT of air, and since 1945 all air in our atmosphere has contained some radioactive articles. Those particles get concentrated in the metal being smelted, leading to radioactive metal. That piece of radioactive metal you're holding wasn't part of a nuclear bomb or nuclear reactor, and wasn't used by Toby Taleban when he was making his dirty bomb. It just happens to exist on Earth, and is therefore radioactive.
So it goes with money. Sure, most money has traces of cocaine on it. So what? So, nothing. The traces of cocaine cannot (well, I'm sure they can, but they should not) be taken as evidence that this particular note was used in a drug transaction, or even that it was ever handled by a drug dealer. On the other hand, the chances that it has rubbed up against another note - most probably in a bank - that was part of a drug deal, or rubbed up against a note that rubbed against a note that was part of a drug deal or [six degrees of freedom]. The money wasn't contaminated because it was part of a drug deal. The most likely explanation is that the money was contaminated because it exists in America. Which is exactly the conclusion on p.5 of the linked doc, and why the authors concluded that pearl clutching wasn't helpful or worthwhile.