$75 is a fair charge, but if you spoke with the mechanic ahead of time and was told that he’d do the diagnostics for free, he should have honored his word. With that new information I’m inclined to agree with Tardrex that his word may not be his bond, and his shop should probably be avoided in the future.
If you showed up wanting other work done you 50/50 would not have been charged. If you show up for one item(Check Engine) you used up mechanic time and garage bay from billable work so typically charge happens.
In all walks of life you will find people who span the extremes of diligence in their chosen field. I know people who would do only the absolute bare minimum as well as people who go way beyond what is reasonable to the extent of being fanatical. Mechanics are no exception.
This guy might be the type of person that expects people to do the right thing (in his mind) and that does not mean simply turning off a light when the underlying cause is not corrected. He offers free “diagnostics” as part of the repair process. Those costs are bundled into the labor rate he charges to recoup the time spent doing real diagnostics.
When someone comes in to his shop wanting to “turn off the light so I can pass emissions testing” but not wanting the problem solved, he consciously or unconsciously puts you into a category of least desirable customer for his shop. Those customers get charged full price or perhaps more to account for the inconvenience and future hassle of dealing with those type of customers.
Putting myself in his place, if someone came in and specifically said they wanted the light turned off, then that is a service not a diagnostic. Certainly, he should have been more clear to the customer about his intentions regarding charges however, just because something took literally minutes to do is not indicative of what should be charged. People often fall into this trap. Hey, it only took you 5 minutes, why should that be $75? Because that took time away from more lucrative work, he has to pay for the shop & equipment and his expertise is worth something.
@TwinTurbo is absolutely right. Turning off the light was the service. You specifically did not want diagnostics; you wanted the light off so you could pass the state emissions test. You got what you requested.