I think you have it backwards. If you commit tax fraud, you will be prosecuted. And the FBI will work with the IRS to do this. But, supposedly, putting a non-z

I think you have it backwards. If you commit tax fraud, you will be prosecuted. And the FBI will work with the IRS to do this. But, supposedly, putting a non-zero value in the "illegal income" field of the 1040 (which ISN'T fraud) both (1) can't be used as evidence against you in court, and (2) isn't reported by default to the IRS to the FBI or other law enforcement agencies, so you don't end up on any watch lists. Of course you gotta take their word for part (2), but it is their incentive to get every tax dollar regardless of source.