question to professional hair stylists about shampoo for colored/damaged hair

I've recently been looking at ingredients more and have noticed that while most conditioners from salons are good, the shampoos specifically meant for colored/dry/damaged hair have sodium lauryl sufate as the main detergent. I've always read this was the harshest detergent and also strips color from hair so why is this in the shampoo meant for color treated hair, especially in salon formulas?