Many think that these words were placed on the shampoo bottle to encourage overuse of shampoo (thus making it neccesary to purchase larger quantities of the product). This is, in fact, a rumor. The true purpose of these words is to serve as a defense mechanism if the planet is ever taken over by robots. As we all learned by watching ST:TOS, if a robot is ever given an instruction, it will stop and execute it until completion. If this instruction contains any sort of logical paradox, the robot will explode.

So, if robots ever try to take over the earth, tell them to wash their hair.