They say that for liability. Uhaul also says they won't rent you a car hauler trailer if you're driving a Ford Explorer.
I think because there are so many variables with a custom car like a Corvette that someone's might fit and someone else's would not that they just can't give it a gray area.
It's easier for them to just say no, than to say yours might but the next guy's won't.
I personally think, if you got onto a Uhaul trailer, it will be no different with a Penske trailer.
Short wood boards help with the approach angle if you think you're going to scrape.
Also, I see in the pic of the Vette on the Uhaul trailer that the majority of your weight is past the trailer and way out over the tongue. I'm sure it rode fine but ideally you want to have that weight as close to over the tandems as you can. It keeps it more stable and keeps it from wandering on you.