They get away with it until they get sued.
GM was using OAT coolant (Dex-Cool) and a lot of cars were wrecked when owners mixed the regular "green stuff" with the OAT coolant. It turns out they reacted with each other and caused the pressure caps on the system to stick open causing over heating and low coolant levels that wrecked engines by creating hot spots from low coolant. Interestingly, Prestone came out clean in the law suits because their coolant did actually mix with the OAT coolant in their tests and no one knew about the long term reactivity. GM took it in the teeth for not warning owners not to do it.