Hair products are big business, companies that make hair products want to make money, the best way for them to do that is to get you to buy more products. How do they get you to buy more of their products? By encouraging you to use up the product you have faster, hence the standard shampoo directions: lather, rinse, repeat.
Unless I'm getting a relaxer touch-up, it is a rare occasion that my hair is dirty enough to require more than one lather of shampoo, even when I'm using my sulfate free cleansers. I've found that if I thoroughly rinse my hair (for at least 2 minutes) prior to adding shampoo and then thoroughly lather my hair the first time, using additional product to get the full lather if necessary, there is never a need for me to repeat that process. One lather cleans my hair just fine, keeps more shampoo in the bottle and more of my money in my pocket!