If it was my shop that might make sense, but they don't. In the case of the OP's shop, that's not how a minimum charge works. I bill hourly and I have a minimum charge of 15 minutes. If a client calls me up on the phone and asks me a question and the whole conversation is 90 seconds, my client gets charged for 15 minutes. But if I do something that takes 19 minutes, I charge for 19 minutes. (Actually 20 minutes because I round to the nearest five minutes, but that's another topic).
If the OP's shop had a 1 hour minimum and the oil change took 15 minutes, he would have been charged for one hour, not for one hour fifteen minutes.