Why force companies to buy pumps that blend when you can force all liability onto the customer?

Gas stations can get away buying cheap blending pumps and if it breaks someone's older car just shrug and say it must have been the previous customer's fault, we're not liable.

It's probably administrative law associated with DOT regulations. So yes it is a law but not quite in the same way you think of when Congress passes a law. Instead Congress passed a law that said DOT we give this agency the power to regulate these specific things. Go create a working committee and create some regulation. Administrative law is a bit more like civil law than criminal law. In general violation is just fine and they are handled by administrative law courts. Part of what makes them so different is they do not fall under the justice department they are contained within whichever agency has jurisdiction over that area of regulation. They've been affirmed to be functionally the same as federal courts, but can only sanction the guilty party in the exact manner the regulation says. Otherwise when the case is concluded and a party is found guilty is then referred to a federal court for sanctions.