Airlines have several odd habits. After they give the courtesy drink they'll immediately come by within five to ten minutes to pick up empty cans. Personally I don't like to guzzle down a soda or it just shortens the time until I have to use the restroom and decreases the enjoyment of the drink. The really odd thing, though, is that they don't come back by again until the end of the flight. If you're on a three and a half hour flight then you'll probably be holding onto an empty can and cup for a couple of hours.