My favorite LBS also has this type of sign on the door and I understand the reason. He operates a very small shop in a lower income neighborhood with alot of kids most on pretty beat up bikes. They like most kid abuse thier bikes by just dropping them where they are. The shop deosn't have room for bikes laying in the floor and leaning the bikes against others in the showroom is likely to scratch or otherwise damage a bike he is trying to sell. When I'm not riding my beater I ignore the sign and the owner never says a word. In fact if I have something new he will usually check it out even if it is pretty low end.
Remember not all customers are as considerate as you are. When enough customers cause enough of a problem the owner has to find a solution.
Craig