A £5 a pint is pretty standard. In my local airport it's £5 a pint and we live in the provinces. That applies wether the airport has paid tax or not (and believe me nobody cares who pays tax to where - Its the catering companies charging the tax and not the airlines.).
In London or downtown Geneva you'll be lucky to get a pint anywhere pleasant for less than £6/7. You also conveniently neglect to mention the £5.20 double branded spirits and branded mixers onboard. You'll not get that in a wetherspoons in London. It's also around £30 for a full bottle of champagne. Or £16ish for a whole bottle of wine. Chain 'restaurant' prices. People will pay that on their twice weekly visits to Frankie and Benys or Pizza Express. Why should an aircraft be less expensive when it has all of the overheads associated with supplying catering on an aircraft.
People drink in airports because they know they'll be limited onboard and pre flight drinking won't stop, even if it was a free bar onboard. It's a chance to tank up pre flight and not have to wait for a trolley service to pass through the cabin and that's before the crews limit their supply or heaven forbid the captain decide to run a dry flight.
The airports have made the pre flight experience into a pub crawl, not £4 tins of warm beer onboard. Should we employ shooter girls selling £1 shots onboard?