Well the response to your question...

I keep the recipt from the shop that ran my exhaust, just in case I ever need to use it in court. If you have a recipt from the shop and you get a ticket for altered exhaust. I would be willing to bet the judge will drop the charge in court. If not, then I would take the shop that installed the "illegal" exhaust to court and sue them for installing an illegal exhaust on my car without informing me that the exhaust they ran is illegal.