There are a couple of reasons. 1) You're carrying someone's property and you want to make sure it doesn't go anywhere on you. II) It can be wider than what you might carry on the back because you can see it to maneuver it. c) It's actually better to weight up the front wheel for handling because bicycles tend to be too rear heavy to begin with. Adding more weight makes it handle squirrelly and can even make it buck while you're trying to mount up if you put too much (say 20 kg) on the back.

Matthew, do you know anyone in Boston with an F-series? I saw one rolling by yesterday in Harvard Sq.