Legal licenses are paid for through your taxation penalty in the negative quality and you are presumed to get them when you purchase the item with your legal SIN.
The risk to your SIN is pretty hefty.
That is SO not what I expected as an answer.. let me try to readjust my presumptions here:
Shadowrunners are expected to pay a tax on their shadowrunning income.. despite it being for illegal services rendered and paid for through untraceable certified credsticks? How would the IRS even know about it? WHY would the Shadowrunner self-report the income?
I'm guessing... "tax" is a loosely used term and runners with legit SINs just have more overhead than the SINless because they have to shell out bribes and such to keep their record looking clean and unsuspicious despite being career criminals?