Wow, what an interesting thread.
I am more of a water person than a soda, surgary drink person. If I'm at a family style restuarant (such as Chili's, Applebees, etc) I usually do order water. Not because I'm being cheap or frugal but because that is what I want to drink with my meal. Now, in the area that I live in tap water (sometimes even when its filtered) tastes horrible. When I order my water, I will usually taste it first. If it doesn't have a good taste, I will add flavoring to it. If the water tastes okay, I drink it as it is. I don't see anything wrong with it. Really, if the restuarant had a problem with it, they should say so.
I also do not drink a lot. I can usually have one glass of water with my meal and that's it. My husband on the other hand is a huge soda drinker. He will get his soda refilled numerous times (3 or 4 times at least and at most places free of charge) during the course of our meal. I would think that his numerous soda's would cost the restuarant more than my one glass of water.
I really don't think that you can compare using a little bit of flavoring in your water to bringing in outside food or beverages. A flavor pack or a few drops of Mio does not compare to brinking in your own bottled water or soda.