In short, Disney places a charge on your card to ensure any incidentals are covered. It's been discussed at length on many other threads, so if you have time you can do a quick search on the boards. You get that charge back, though it behooves you to ensure you get it back when your statements comes in. It's not popular with guests, but Disney has been doing this for some time, so it's not a scam if you're worried about that.