Chapter 1065 - IRS

Important reminder for everyone: Income of any sort, even illegal ones, are taxable. Don't forget to report anything you've earned from fencing stolen goods, selling drugs, and any bounties you've collected. Not even the Joker would mess with the IRS so you shouldn't either.

The reason this rule even exists is that it's sometimes easier to convict criminals of tax evasion rather than whatever other crime they might have actually committed.