Topsheets have passed out of common usage and so some people don't understand what the top sheet does. I'm English and I prefer a top sheet between my body and the duvet cover. Nevertheless I wash everything everytime so it doesn't matter where my guests sleep.
I reckon if you told your guests: oh you see that coverlet on the top of the bed- I don't wash that each time, they'd be so quick to use the top sheet and ensure the cover doesn't touch any part of their body.
I find it cray cray having a cover on the bed that isn't cleaned between guests. Yuck.