Why is it looked down upon to order water at a restaurant?

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I basically never order alcohol, and I'm pretty poor so I try to be frugal when I'm eating out. I'll never forget going to a restaurant one time with some friends, ordering water, and having the waitress keep "forgetting" to bring it. (My friends ordered drinks and she brought those, of course.) Then at the end of the meal, when she brought the bill, she said to me: "I guess you didn't need that water, after all, did you?"
The owls are not what they seem.