German bus services are not publicly owned anymore and as private companies or conglomerates of companies they are free to implement whatever payment methods they see fit. I personally cannot speak to Nordrhein Westphalia or this particular bus company serving the airport but I had a similar experience in another state. The fare is significantly cheaper via smartphone (which refuses to install on Graphene) and the busdriver in question had no means to handle cash.
Ticket machines are still available, but only in highly populated areas or on locations like central (railway) stations. All others have been eliminated.
Ticket machines are still available, but only in highly populated areas or on locations like central (railway) stations. All others have been eliminated.