Just a few facts about pouring a glassful vs. giving the can..
1. It elimnates waste... throwing out cans that aren't empty.
2. It enhances customer service because the flight attendants are visible and available offering seconds.
3. The more cans that they have to board, the more weight. The more weight, the more fuel used. The more fuel used, the higher the ticket price. I think you get the idea.