Didn't know that...just assumed it's a cost of business like website hosting fees..
But what I don't understand is that those same shops use the online service to 'regulate' passenger loads also. It's won't allow a booking if the max capacity for the trip is reached. The boat captains also use the booking service to generate a passenger 'manifest' which is a legal requirement.
So if a shop goes around the booking service and does 'off-line' bookings, I wonder how they get their manifest and also generate their customer profiles & sales history. I've got no stake or claim in the booking industry, but it would be interesting to understand how Off-Line booking sales are tracked without doing double work re-enter the data in a 2nd place. I'd guess there is something in the contract about off-line transactions that don't get paid a booking commission.