Because you let them. When I use to buy the phone in carrier stores, I told them before they even got the chance "don't even open it, I'll set it up at home"....
I still have the ability to cancel. Placed an order on 2/21 at 7am with an estimated ship date of 3/4 and delivery of 3/6. None of the added accessories have shipped and none say backordered at this time.
I've used speck, dbrand (obviously), official Samsung rugged, and several other brands that never damaged my skins. I've had skins on 4-5 of my last phones with cases.