It isn't that we KNOW people are trying things on without underwear, but that we don't know they aren't. I mean, if you have to include a note to tell people to try on WITH underwear, it seems logical to me that there may be some people out there who will skip the underwear, whether they're sure they're going to keep the item or not.

I'd rather take the extra time to wash my new shorts before wearing, just in case.