If you look at 171.1, it says these rules apply to people who transport HazMat "in commerce." The best explanation I've heard is I can go down to the gas station and fill a gallon can for my lawn mower and drive home. If I slap a "Viking Lawn Care" sign on the side of the truck and drive around town mowing yards for hire, I should have a class 3 placard on the side of the truck.