I'm about to take one of my cars in for my biennial emissions check and it's become a habit for me to reset all trouble codes and clear my history 30-60 days prior. Why you ask? First of all it takes about that time for the ECU to rerun all the necessary internal systems checks to pass the emissions tests after a reset. And because emissions checks are now done using the OBDII connector and the Illinois state inspection stations download your MIL code history every time you let them scan your car. How do I know? Because a few years ago a tech let it slip that my car had certain coded in year XXXX and other codes in year YYYY, but they were all clear now. I remember this incident because those were years I didn't own the car. And I know they were cleared soon after I bought the car and again when I retuned the ECU and TCU for the added blower, exhaust, and tranny shift kit.
But why do they need to collect that info? If I pass, I pass. Anything else is none of their business.