That's the only reasoning for this that I can think of: Darth Vader is so desperate in the wake of the economic downturn that he has resorted to armed robbery just to stay alive. It's kind of odd though that he uses a handgun when he could have just used a Jedi Mind Trick to make the teller hand over the cash.
In other news, it seems that some folks at a Comic convention are in need of a little guidance, as stabbing someone in the eye with a pen because they're sitting too close is something we would have hoped would be left behind after grade school. I guess the pen is pretty mighty, but I'm still not all that impressed with its utility as a weapon; unless of course we are talking about the new tactical pens that cost more than a good folding knife.
And while we're on the subject of the utility of weapons, have y'all been watching the show Deadliest Warrior? Super cool show. I see some things that might not stack up in the really real world, but overall it's a neat show. You've got some smart guys who pit two different warriors from throughout history against each other, and do scientific tests on the weapons. The effects are horrific. Check it out some time.