I had a similar experience with Discount Tire. My Finacee's Benz needed the tires rotated and balanced and since I bought the tires from them it was free. Her car is similar to ours because of the staggered widths. I always watch the employees (when I am able) when they are wrenching on my cars. I watch as this guy proceeds to move the rear wheel/tire to the front and visa versa. I stopped him immediately and corrected him.

Lesson here. Always watch the workers when you have the chance. They will notice and therefore be more careful and you just might catch them making a mistake before it becomes a problem.