TJW - I'm not sure I understand why the fact that the ticket has to be purchased from the airline implies that the airline is paying commission to the railway company. It would be much too difficult for Deutsche Bahn to check whether passenger X has a ticket on date Y from airport Z - if only because of passenger confidentiality rules. Only the airline can know if someone has purchased a plane ticket. Thus, if Deutsche Bahn wish to limit sales of this ticket type to genuine air passengers, either the passenger has to show some sort of printed out boarding card at a train station (as happens with trains to/from Prestwick) or the airline sells the rail ticket.
Separately, easyJet and Ryanir are more than happy to sell a rail tickets to/from places like Gatwick and Stansted at prices lower than the walk-up fare... (and I doubt MOL is doing this out of the kindness of his own heart to passengers, paying NationalExpress a commission in the process !) - so why not do the same thing in places like Berlin or Bremen as well ?