I'm not sure that they'll just flat out rip you off by and large (yes it does happen) but I think you see a lot of "pessimism" in their evaluations. A part that will wear out after 50,000 miles might be recommended for a change at 42,000 that sort of stuff that basically turns into a form of upsell on services on the date of service.
Along those lines, a lot of the things that are "recommended" to be done periodically won't have any impact during the time before the owner trades out of their current vehicle, so they are, to that person, a wasted cost in the eyes of that owner. Of course, this is basically the tragedy of the commons, but still a valid choice by the owner and source of frustration.