Found the IRS publication link:

https://www.irs.gov/publications/p17#en_US_2021_publink10001...

See also kickbacks, slightly lower down.

Has to be mentioned explicitly despite often, but not always, being illegal.

This is probably misleading as most people present it. The IRS does require you to report illegal earnings. But the justice department can also use that against you[0].

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...

Paying tax in and of itself is never illegal. Paying more tax than you owe is not illegal either. As far as taxes go, the only illegal thing is “not guessing the number right” and flat out refusing to pay.

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.

>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.