I don’t know if it’s different in the US, but here in Europe you can bring water bought after going through security. I think it’s more a means to make people buy things at the airport at inflated prices.
You can buy water in the us airports, or bring an empty bottle and fill it. And besides they HAND you a cup of water on the plane if you ask. Had nothing to do with making a "mess" not did it have anything to do with forcing you to purchase anything. It is merely because it might be liquid explosives.
