Why? The example from Piskvorrr is textbook violation of 18 U.S.C. 473: (paraphrasing here) Handing out fake money with the intent that it believed to be real is a crime.
- mark gets an offer of a great exchange rate from the con-man
- mark hands over real money
- gets a stack of something - the first few bills on the top and bottom are actual local currency, but most of it are one-sided counterfeits.
- con-man counts the money in a way that doesn't give this away - this obviously requires sleight of hand and insufficient lighting
- con-man disappears in crowd
- mark eventually realizes that most of the stack is worthless
The whole con takes a minute or two at most: that's not very long at all.
The critical section "mark no longer has original currency but hasn't yet realized he has been swindled" is as short as a few seconds; this has even been done with plain paper in some particularly audacious cases.