You'd have to raise that with the caterers. Its their products the airlines sell. They take the money. It's their POS devices used to process payments. The airlines just take a commission and don't decide where tax is paid or exempt. The prices are obscene in your opinion. However I bet a double branded spirit and branded mixer will be a similar price in a city bar, and compared to downtown GVA are pretty cheap.
If people want to drink, they'll drink regardless of the cost onboard. A stag do won't arrive at an airport and think 'let's look at at the nice things to buy in the shop' or 'let's sit down for a nice green chai latte' knowing that there's cheap drink onboard. I doubt the onboard prices even crossed the minds of this lot. They drink at the airport because they want to get drunk and know there's very little chance of the airport turning down their request for drink.