It's not meant to deter "real" (professional) counterfeiters -- this, along with the copy machine code, is meant to stop casual counterfeiting. Which is a good thing, as there are quite a number of people who are normally honest, yet could be overcome with a strong enough temptation (getting a free $20 bill), if it appears easy enough.

And of course they will get caught and have their lives ruined. So by making the act of falling into the temptation a bit harder, you are keeping more honest people honest.