For me ... it's because I'll drink champagne that is much more expensive than I drink at home. IN BA F for example they sell Johnie Walker Blue ..... I'd never stump up that. They are serving champers that is £100+ per bottle.
I know you'll say that if we can travel F then we can buy expensive champers but not all people are like that ....... my champers tops out at £35 per bottle and is normally around £15.
I still don't get it. Even if I'm not going to keep my fridge stocked with bottles of dom, i'll have 1 glass pre-departure, then switch to water so I'm hydrated and wake up at my destination refreshed and not hung over. Just because free booze that's normally out of your price range is available doesn't mean you have to clamor to drink them dry.