Many people don't know that you can't just sell anything out on the street and claim it's a fuel additive to put in your car.
Speaking of snake oil, this EPA approval process isn't really about the EPA verifying that an additive does what it claims. It's much more about making sure an additive won't destroy your engine. The EPA looks for unusual things in the formula which aren't normally found in gasoline - stuff like heavy metals that would cause engine damage.
What happens if somebody puts together a witch's brew and starts selling it as the latest and greatest fuel miracle without first registering it? If they get caught, they're looking at big fines or even jail time.
Mix-I-Go has always been "EPA-registered" - just another reason why drivers have put their trust in Mix-I-Go since way back in 1927.