It's much easier to build a tax case than a criminal case. In a tax case the IRS/Justice Department can do an estimate of your income from assets and purported lifestyle and the courts will buy it whereas a criminal charge has to be proven to a much stiffer standard.
If you go to a strip club and nurse a beer for 3 hours, the IRS/JD can portray it as an evening of nightclubing that cost you $500 and get away with it.
Just like the IRS tells waitstaff how much they make in tips and they have to pay on that regardless of what they actually made unless the waiter/waitress makes a federal (literally) case out of it.
