Brilliant, when you think about it.
https://www.irs.gov/publications/p17#en_US_2021_publink10001...
Has to be mentioned explicitly despite often, but not always, being illegal.
There’s quite a bit more case law on the matter, but tl;dr is that it’s a very fluid and ill-defined area of law. Personally I expect tax returns to be used as evidence more in the future, rather than less.
0: https://scholarlycommons.law.wlu.edu/cgi/viewcontent.cgi?art...
Obtaining money through criminal activity is a crime, but that's a different crime that does not have anything to do with paying taxes.
It is a crime to knowingly accept money obtained through illicit activity, but guess what – the prosecutor for that crime is the government itself! You can't sue the government for unwillingness to prosecute.
No but selective enforcement could invalidate enforcement of the law, per equal protection clause of the 14th amendment.
Funny that the US government had no problem believing the money was legit when it came time to pay taxes.