My experience of Dixons retail was that they were cheaper than a landside store.
To be honest if people see a price and are happy to pay it, I don't see the relevance of the whats and the whys regarding what happens to their money once it has been handed over. If they were happy to pay it then what's the problem?
I always thought they asked for boarding cards so that they can see which flights make the most money which is important when it comes to negotiating handling fees and the like and not taxation.