It's effectively two different book rates, the warranty book assumes a new car. The non-warranty book assumes an older car. Older car = rust, rounded fasteners, and other things that add to the time.

Mechanic gets used to working at non-warranty rates, so any warranty works feels like being cheated.

Does it all balance out in the end? Maybe. But, that's not what mechanics "feel" like happens. And they're paid little enough (generally) that the two books annoys them. And the working conditions are bad enough (both physically, but also "mentally" - many shops are hostile workplaces, or close to it, with poor benefits, long-ish hours, etc). So, they have a lot to bitch about, generally.