Many UK airports have a "single till", aggregated income, concept - so if the passengers of an airline contribute to the ground revenues of the airport by spending a lot in the shops, the airport reduces the landing and handling fees for the airline.

I'm not sure the aggressively cheap airlines would want their airport fees to go up if the passenger airport spend, on alcohol, was reduced.

Of course, checking all that airport-purchased alcohol in the hold would prevent it contributing to any inflight problems. The topic of other problems caused by airlines discouraging bag checking has come up before, I believe