Good point. The way the law was originally written (1970, thank you President Nixon's war on drugs), banks had to report cash deposits and withdrawals over $10,000 to the Government. Courts generally held that it was not illegal to take out $6,000 on Monday and $5,000 on Tuesday to avoid the reports, so, in 1986 (thank you, President Reagan) the statute was amended to make it illegal to "structure" a transaction in order to avoid the reporting requirement. In 1994, the U.S. Supreme Court held (Ratzlaf v. United States, opinion by Justice Ruth Bader Ginsburg) that you were not guilty of "structuring" unless you knew that structuring was illegal; the law was immediately amended (thank you, President Bush) to make it illegal to "structure" even if you didn't know it was illegal to do so.
Each of these laws, incidentally, was justified by the need to detect drug dealers.
You might want to double-check who was president in 1994.