1) You technically lose money when you pay for any type of entertainment. Do you consider it a loss when you pay to go to the movies or a sporting event?
2) The possibility of hitting it big is one of the attractions of casinos and adds to the excitement. Again, I said if you gamble and expect to get rich, you are an idiot, but the possibility is there. Many people leave Vegas with much more money than they went with. This is simply offset by the even larger numbers of people who lose money.
3) No reputable casinos would ever ban winners and hope to stay in business very long. They don't have to. The house has the odds. The casinos care about getting gamblers to the table and keeping them there. Again, for ever big winner there are a few big losers. If people found out that winning would get them kicked out of a casino, no one would go there.