This is true for a lot of US resturants (especially fast food) they don't have a button for water, instead they just ring up a soda, and fill it with water.

I think the majority of the cost is the packaging The concentrated soda and carbonation isn't free, but a tiny fraction of the cost of the cup, lid and straw.

My understanding was that restaurants in the US are required to provide free tap water. It may be a very small cup, but it should be available.

You get the water, but you can't take the price of the soda off the menu item.

Say you get a "Big Kahuna Combo" at $4.95 from some joint. The combo comprises a hamburger, a bag of fries and medium soda. Let's say the supermarket price for the same amount of soda is $.95 and the water cost is $1.95 (yes, more.) You'd think the restaurant would be able to swap the soda for a bottle of water and charge you $5.95 for the combo. But the way their combos are created (there are many, many reasons why the combos include a soda, such as soda companies subsidizing the cost of the soda for having soda as the default drink in the menu) doesn't allow for customization. In that case you'll have to pay the full value of the combo + $1.95 for the bottled water. Or get the same combo, for $4.95, but replacing the soda for tap water.

That's what the parent meant.