Con de and other absurdities

I have been in Xalapa for the last couple of days and encountered an interesting change in Spanish grammar.   I was at breakfast yesterday and a friend asked for "un vaso de agua."  The waiter corrected him and said "un vaso con agua?"   Evidently there are some in Mexico that argue that the word between glass and water should be "with" not "of."

There are five meanings of the word "de" in Spanish.  De can mean pertaining to,to contain, the origin, authority, or source.   Vaso con agua does not sound right and it also does not make sense to me.  You want a glass WITH water?   If you carry the concept to its logical end you would get some odd constructions.  For example, the University that I spoke at today is called Universidad Anáhuac de Xalapa (the Anáhuac University of Xalapa) using this logic the name would hence force be changed to Universidad Anáhuac con Xalapa (the Anáhuac University with Xalapa).   The other problem of using con is also conceptual.  Often when I order Whiskey I ask for Whiskey con agua.   In most bars that either means a glass of Whiskey which is mixed with water or as WC Fields used to order it, a glass of Whiskey with a side of water (he claimed to wash his hands in).   Often to clarify, I ask for Un Whiskey, de recho, con agua.  Which means a whiskey straight up with a side car of water.   The waiter's insistence on con seems like a whole bunch of nonsense to me.