Perhaps we should do like the British and, to avoid double taxation, only charge sales tax on outgoing flights to the UK. Flights to England from US border airports are usually more expensive, so additional tax shouldn't be an issue. It might encourage people to visit France instead of the UK, but that is what the British are doing with their discriminatory anti-Canada tax.
By the way, if you spend 24 hours or more in Dublin, Reykjavik or Paris on your way back from London (or book you segments separately) you avoid much of that $106 air duty. Obviously, you want to avoid flying out of Belfast UK if you have the extra 2 hours to get to Dublin, Ireland, by bus.