It's often not offered because it sounds like asking for a free drink instead of paying ~13 euros per liter which seems to be the average norm. Those that offer tap water (that's most places) will usually (not always) charge you the same as a normal water, which in turn is the same price as any soft drink (demonstrating that the price has nothing to do with cost, it must be paying for the table or something but it doesn't say so I don't know).
Anyway, it's more about places charging dishonest prices to seemingly cover for something else, leaving the consumer to guessing why in the world this might cost so much and just going with another option. I can't imagine it achieves the intended goal, so it doesn't seem like a good idea for either side. If anyone ever did finances for a restaurant or has a better-than-guessing idea why SSDs are priced this way, I'd be very curious.
I don't think I'll be able to make sense of what you're saying until you explain what you mean by "normal water". Are you referring to branded bottled water?
If anyone is interested in unusably tiny and/or slow SSDs, let me know because right now they're just going in e-waste.
(I have similar beef with drinks in restaurants at, e.g., ~100x markup for tap water. Why not just charge a normal price for both the food and the drinks, instead of me having to guess at how much I should be spending on drinks to compensate the normal-priced food? Or make both cheap and charge a table fee, whatever floats their boat. This incentivizes people to not drink enough; usually it's calories where people overingest, not hydration!)