If one doesn't want to understand one will not understand even if it is as simple a this: An ICE ticket is generally not valid on trams, skytrains, subways and busses.

Translated into a New York experience: An Acela Express ticket is generally not valid on the port authorities busses, ferries, subway or the JFK Airtrain.

Or translated into a London experience: A Heathrow Express ticket is not valid on the Underground. A Southern Railway ticket is not valid on a Heathrow Express service and a bus ticket will not get you on the Underground.

Everybody considers this as normal in New York or London and here we see a lengthy discussion why people do not fancy it in DUS. I personally would decide to install fare gates and make everybody without a VRR ticket or pass pay 5 EUR (like for the Airtrain in NY) Good for me as a taxpayer and a passholder. And it clarifies all "mud".

BTW: Not presenting a ticket if requested will cost you 60 EUR on any no-long distance train and might result in criminal prosecution.