If the customer's tires were bald ( 2/32" or less), the shop could be found negligent by a court if they rotated them. The bald tires were probably in the front (front wheel drive cars will wear those out far faster than rear tires), and putting them on the rear would lead to a spin out in the rain. If the tires were all 6 years old, that alone is reason to replace, as the tread gets hard with age, cracks and oxidation take their toll.
Some people are just too cheap to do the right thing.
Tires are not the place to scrimp, as they're the most important safety item on a car.
Professional race car driver for 25 years.