Well, you have been required to show your ticket to bus drivers in DUS for a couple of years now.

Agree with your point about it being a philosophical issue, but also agree with others that the Skytrain issue is pretty confusing. It really is a German thing to have a rulebook about which ticket might or might not be eligible for a free ride on the Skytrain. Especially since the English website of DB sells the nonstop ICE ticket from Cologne to "Düsseldorf Flughafen" - not Fernbahnhof, Jamaica Station or anything else. Reasonable for an unsuspecting foreigner to believe it's part of the actual airport.