If someone just gets blitzed and makes a fool of him or herself, then the mere inconvenience and expense of being taken off the plane is enough.
Do you think it's free to just land the 'plane at an airport? What about the 'inconvenience' to everyone else on the flight, staff, passengers, people trying to make connections, meeting family, doing business where they land....
If you are drunk blah blah blah who sells them the alcohol in the first place?
They could have easily had enough prior to take off or before they even checked in (never mind carrying on in th' 'plane) - it's how they handle any alcohol and what respect they show to others irrespective of what they've drunk that is the issue.