Ashman's comment about it being easier to keep old customers than to acquire new ones is well taken.
The incident in the original post reminded me of a series of similar experiences I had with my dealer over 20 years ago. (Will things never change?)
The result was that I bought a set of the MB manuals, some metric tools and a few of the MB special tools and began taking care of my car myself. Not only did I know the work was done properly but I saved a momentous amount of money. In trying to rip me off, the dealer lost a customer and most of my business. I have heard it said that owning an automobile dealership is a license to steal. Many of them seem to subscribe to that code.
Thank goodness for competent, honest independent technicians. Keep up the good work, guys!