The law doesn't consider the HazMat trailer loaded after all of the product is delivered. The law simply states placards stay on until it is washed out. All laws are not constantly harmonized in light of new laws or changing situation. One law say what it says another law says what it says. They don't have to make sense and common sense isn't the final judge of their meaning. Think of each law having a history, starting before it was passed, and then rulings from judges in a particular lawsuit, etc. The lawyers & judges fight over each word and each situation & definition are complicated because in situation A it was ruled, appealed, and then upheld or overturned it was determined it meant exactly this.
Law School would be about 1 or 2 semesters long if laws meant what the dictionary definition of each word meant and then logic and common sense smoothed out the rough edges. Law school is 3 years because there is a lot more than a dictionary & a logic class to know.