🚀 go-pugleaf

but it's not unusual in the printing industry to print on transparent materials

You are right but in this case, a "paper" background is a mental concept. In indesign, it simulates the render of the medium. It's not a ink.
You can have a "paper" background set to blue. It won't mean you will export blue datas but it helps the designer to make his layout based on the colored paper onto which it will be printed.
"Paper" looks white because most of the time, jobs are printed onto white paper. But once again, this is not a real data but a help.
You can have a white paper background but when you will print on a transparent media, there won't be any white ink on it.