Why Is Restaurant Wine So Expensive?

I know, I know, we've had this conversation before.  Wine is over-priced at restaurants.  But recently it became REALLY obvious.  A couple weekends ago my wife Karla and I went out to a place we'd never been before.  It's a nice place that on average would cost a couple close to $100 for dinner.  Normally, I'll order a mixed drink and Karla will order wine that's normally $10- $15 a bottle in the store.

So that night she ordered a a small glass which, at other locations would be a quarter of the glass.  At this place a $6 glass of wine was literally 3 sips.  I could've gotten a 22oz beer for the same price.  What gives?

Restaurants sometimes triple the cost in relation to the bottle and more if you go with what they paid for the bottle. Don't get me wrong.  They should be allowed to make a profit but the amount of gouging is so obvious it's insulting.  The reasons that's given most often are first, they take the time to select a wine list to go with your dinner.  They may also give advice as to the wine that goes best with what you're ordering.  Then there's the fine glassware and they open the bottle for you.  Does this validate the mark up?