A lot of the time the "advice" people at the dealerships will give is mostly to protect their own *** (although yours as well to an extent.) It's the same as why Timmies has to put "Caution: HOT" on their coffee cups nowadays: They've had people try to sue them for their own riding mistakes before, hence this necessity. I've had a few sales reps from a few dealerships admit this to me once getting to know them a bit better.
Mostly advice like this, or not exceeding certain RPMs etc is to get a new rider (or a rider on a new-to-them bike) to take the time to adjust and learn the bike before they start pushing it. Does make some sense really, all you have to do is look at some of the YouTube videos of people crashing their brand new bikes to get a sense for why this advice is given.
When I first took my girlfriend 2up on my 250R a few years ago, I had been pretty confident in my riding capabilities even after only riding for about half a year. I learned quickly how much taking a passenger changes dynamics, especially on a smaller displacement bike. Took the better part of that entire day to adjust to the difference. It still takes me a few minutes from time to time to readjust, the odd time she's not on her own bike and rides with me.
That said, yes, some of these people really are just talking out their ***.
