Tech probably just screwed it up because flat rate techs are always racing the clock.
If you were just dealing with the service writer the tech probably never even knew you had issues with it until the situation was too far gone to salvage.
Even if the plug was improperly gapped from the factory (or dropped on the floor) the tech would have to miss that and the fact that the car ran worse when he finished than when it arrived. And that the computer said things are worse.
It's either malicious or so incompetent that there's no practical difference.
I talked to the tech on the phone before finding out about the plug. The tech definitely knew I was having trouble. Interestingly the tech did admit it would be possible to bend a vale doing the media blasting. I guess that's why service writers exist to "manage the customer".
When I got it back the check engine light was on and the car was idling rough where previously it was just notifying of scheduled service and running fine. The dealer took another look and determined that I had a bent intake valve. They quoted me $15,000.00 to fix it.
They claimed this must be my fault, probably downshifting on the highway. I noted that the car ran fine when I dropped it off and a missed shift would bend more than one valve. Also, they were just doing service in the intake near the valves (with a media blaster) and that the car had 30 more miles than when I dropped it off. It seemed obvious to me that any engine damage was done by them. They insisted it was "impossible" that the damage was done by them.
I said not to do anything and that I would tow it to another shop for another opinion. Strangely they suggested I could drive the car to another shop (with a bent valve?). I had it towed and the other shop determined that one of the new spark plugs had the electrode pinched shut so it was shorting out. They also noted that the coils did not look new. They installed a new plug (~$12.00) and the car ran fine. To be sure they also did a compression test and found no evidence of a bent valve.
I went back to the BMW shop and got all my money back for the service. The service writer still had the balls to tell me that I "should be prepared for expensive service bills on these cars". I wonder how much of that reputation comes from shady dealerships ripping people off. I suspect quite a bit.
I have no proof but I think the dealership sabotaged the car on purpose. There's no way any inspection would have missed the shorted plug. The car was immediately warning of the condition after startup and was misfiring badly.
Dealerships absolutely will rip you off if they think they can get away with it.