I guess in all walks of life you'll find someone willing to screw you out of $8 bucks.
You pay a lot more than $8 for service. I am pretty darn sure that he wasn't even opening the regulator up. He was just holding it for a while, returning it, and sending in a bill.
In Colorado, the overwhelming majority of divers dive only on an annual vacation. Regulators can go for years at that rate without servicing and still perform well. A customer would have no way of knowing from the regulator's performance that it had not been serviced. Our shop eventually realized that was happening on more than a few occasions--my case was not an anomaly--and stopped using him. He still "services" a lot of regulators in our area.