In general: Beer is considered "food" that has to be available to basically survive, no matter what dangerous the situation might be.  (Not that 1 liter of beer would have a noticible effect on a German soldier.) 

Germans drink their first beer with 10 - 12 and regularly when they are 14.  When they turn 16 they can officially buy it. Summed up: They are used to beer consumption - the basic rule is that 2 beers after a hard day of service are allowed and good for the morale of the soldiers.  That rule is kept in foreign missions, and I have yet not heard about huge cases of abuse - contrary to Germany itself where stories of heavy drinking parties in barracks are legendary and everybody can tell you one.