So explain how a dealership can consistently sell vehicles under invoice. They can't without making it up somewhere else. That somewhere else is the service department. The dealership is one entity, the owner wants profits, that comes from service.
You are right about one thing, the mechanic could care less about sales but, the mechanic doesn't talk with the customer, the service writer does. Keep in mind, if the dealer is not busy, the mechanic is going to be looking for jobs, so, anything that looks wrong is a major problem that needs to be fixed.
Also, I wouldn't be surprised if some dealers gave their mechanics a bonus based on how much extra work could be written up. However, the big thing mechanics might receive a bonus for is CSI or customer satisfaction index. The more happy customers, the better their bonus.
I've said elsewhere, my dad has been a dealer mechanic since before I was born, at lot has changed since when he started. While I haven't followed in his footsteps, I know enough to not get screwed and have been told that even with no formal training (I never really took interest in the projects he brought home either) I'd do a better job than some of the recent hires at the place he works (not Honda and not the same city as me).