It's not "intra aiport" but from the terminal to the mainline station. Not anyhow different from JFK: Sky Train from the JFK terminals to the subway requires a ticket (and even subway tickets are not valid).

Seriously: The line was not built by the airport but with taxpayers' money. Therefore it is part of the public transport system in Düsseldorf. I would expect that 95% of all users have a ticket anyway, because they come either with season tickets or from local transport trains - but some are left without. The most important exceptions are:

• pax using the (pretty cheap) Q-Park parking lot and
• pax arriving on a long distance train ticket without "city option".

I assume that there are by far more pax without a valid ticket departing from the airport the mainline station: I hardly see somebody buying a ticket on the platform when heading to the train station (they buy their tickets over there). However: Travelling without a ticket may cost 60 Euro....