The restocking fee is used for people that return systems with no reason. Alienware builds these machines to order, so when you return one they have to dissassemble it, and they can't use the parts again in a new system as they're used.
According to what I've read on their forums, if you have a legitimate problem they can't resolve, they'll take a return without the restocking fee.
Many people though order one, and for whatever reason decide they can't pay for it, or don't want it after a few weeks of play. If AW accepted returns for no reason, they'd eat a lot of labor due to these people's inconsideration.