Any reputable shop charges for doing diagnostic. Their mistake was that, not to explain that this is a complex procedure that requires the use of very expensive equipment and competent technicians. The shops I know that do diagnostic properly and have an ecxellent acuracy record charge around $150.00 to scan and do preliminary diagnostic. sometimes there is more time needed to solve a hard problem.
I found out the other day I was buying some parts for an RV generator at a local small engine repair shop where they fix mower, generators etc that they charge $90.00 per hour. Think about the amount of equipment, training that they have to have compared to automotive technicians and you must see something wrong with that. I think they deserve what they are charging, but our industry have let the people underapreciate us in many ways. WE have to study a lot to work on modern vehicles. The equipment is very expensive and we need to have above all experience. So I think you are not being fair completely with this post. That shop made some mistakes yes. But you can not expect to have a car repaired properly paying peanuts.