Say you need a camcorder to film your niece's wedding. What people do is buy it the day before, use it for what they need, and then return it when they're done. So basically they get to rent it free of charge. Stores lose money every time this happens as they can no longer see the item as new. Best Buy is full of these items. When I bought my first iPad, it was an open box. It was obvious the thing had never been touched. Got a good deal on it that way.
Another thing people do is take an item home to try it before they buy it someplace else. That's how they got their reputation as Amazon's showroom.
Long story short, this is why stores started charging restocking fees. With Best Buy bleeding money, I'm not surprised they took it to this extreme. Though it's unfair to do it for exchanges.