Here are three other thoughts
1. Restaurant owners are plagued with employees who feel generous with the owners inventory, especially liquor and often pour their friends free drinks. a friend who owned 3 restaurants said this practice ate heavily into his pockets.
2. If a customer is going to buy a glass of wine he/she will come out economically better off buying the entire bottle. In WSJ section on wines the author said that a person buying wine by the glass is really paying for the entire bottle of wine
3. I agree with the OP in this specific situation. But by nature most people watch to determine if they are being treated equally
but only a few complain
