I think there are a few factors at play:
* If alcoholic beverages are "free," then the passenger is getting something with a known value, and may see this as essentially discounting the cost of his or her airfare. (I put "free" in quotes, because, of course, you're paying for the beverage in the cost of your ticket.)

* Many people do get stressed while flying and alcohol calms their nerves.

* Many people drink to unwind once they're done working, have put the kids to bed, etc. Time spent flying is down time, and they're having a drink just as they would if they were at home.

* For some, a plane trip is a celebratory occasion, and as with many kinds of celebrations, it's commemorated with a drink.