More expensive bottled water
For patrons who would generally much prefer alcohol
You have tap water in your hotel room, and it's free, isn't it? Ice available somewhere, too, usually. Free unless you want room service.

There are various brands and sources of fine mineral waters from wells and natural springs, and some of them are quite expensive.
There is taste testing and mineral analysis done on them, too.
Most of the clientele of these places complaining about expensive brands of mineral water wouldn't hesitate to blow $26 on a bottle of cheap wine.
You're drinking what? Water! In a very risky town, at that. There's staff and security and the hotel is liable if the said water is spiked or contaminated.
Water straight out of the faucet is occasionally "snowballed" with a poorly soluble mind-altering adulterant drug inserted into the screen or filter of the faucet and left hidden there.
Clean tap water isn't going to hurt you or make you sick, but it is a little bit sulfurous and it may not taste very good in Las Vegas if you aren't used to it.
In a city of gamblers, pimps and drug dealers, ...