There is immense pressure placed upon cabin crew to generate on board revenue. The airline has crossed the line if they are arming their crew with cheaper promotional deals on alcohol.
Crew sell more than alcohol, and alcohol doesn't attract the best commissions in any case, so pressure to make on-board sales doesn't equate to pouring alcohol down passenger's throats!
As the crew are the ones who have to deal with the "problem drinkers", believe me they are well aware of the problems of serving too much alcohol and are usually the first to stop service.
So whilst it is illegal for the airline's customers to be intoxicated on board the aircraft, they are willing to ply them with cheaper alcohol.
Actually the law sanctions
boarding an aircraft while intoxicated, which may sound pedantic, but it means that (as in pubs and the like) the responsibility for serving the alcohol lies with the person serving it. It is also the reason why consuming your own alcohol on board is forbidden.