I’ve never seen soda water offered at a restaurant soda fountain
Look closer next time. There’s two tiny tabs on two of the spigots next to the main buttons, one which says water and the other says soda. It’s very easy to miss if you’re unaware of the options.
They’re not always there.
A lot of times the water tab will be on the lemonade/fruit drink dispenser and the soda tab will on the Sprite/7-Up dispenser.
I’ve never seen it either. I think you’re generalizing.
To be honest, I was shocked when I learned that people actually drink it. I thought it was for spraying people in the face or mixing.
Around here they are going to fancier machines.
example
One Big push button and a touch interface for details. It’s kind of cool because they have 10 or 12 base pops, and cherry or cherry vanilla etc.can be added to any of them.
Those machines are definitely the future. And they have plain soda water as an option. A hard to find option, but an option nevertheless. The person I was replying to had apparently not seen modern installations; I didn’t want to future shock him too badly. 
Moving over to CS.
No it isn’t.
I’m using prices from WebRestaurantStore, a restaurant supply company.
5 gallons of off-brand soda syrup costs a bit over $30. Soda syrup is mixed 5:1, so that means that a gallon of soda costs about $1. The average soda cup is around 16 oz, so the syrup costs about $0.12 (and that’s assuming no refills). There’s probably some volume discounts for major chains, but even bulk corn syrup isn’t that much cheaper.
In contrast, 16-oz paper or plastic cups can be gotten for about $0.03 in bulk.
You can buy CO2 for about $1/lb, and a pound of CO2 will carbonate about 15 gallons of soda. So the CO2 in a 16-oz cup costs less than $0.01.
I very seldom eat at the same restaurant more than once, so it would be 15 percent of establishments, based on a one-time sample.
Just ask, it’s pretty reasonable.
I just remembered, in Farrell’s Ice Cream Parlor and Food Emporium, there is, prominently displayed, a sign offering a glass of soda water for the paltry sum of 2 cents.
So, it’s a thing you pay for, I guess.
It varies. My nearby sports bar/billiard parlor/restaurant charges nothing for soda water.