If it's true, blame the reports of drunkenness and abusive passenger behavior on airplanes.

And honestly, in the world we live in now, it's probably a good idea. Anyone who is truly dead-set on getting drunk while flying has plenty of opportunities to do it, before coming to the airport, in the terminal, at the lounge, with a PDB, and then in-flight. The airline can't really stop anyone from doing this. But it never hurts to slow people up a little bit, and if this sort of policy prevents or defuses one incident in the future, it's worth it.