Bill, it's a complicated issue, and it's been debated both here and on WLDG (and in lots of other places, I'm sure).
The bottom line is that the restaurant business is a risky, unpredictable business in which creative people (and their backers) operate on thin margins. A number of standard practices have evolved to make the numbers work. One is tipping (another topic for a separate hot topic); another is looking to the bar - speaking more broadly than wine alone - as a profit center.
Yes, restaurants expect a significant share of their income from wine, beer and liquor, and yes, there's a high markup on a product that requires relatively little value added. You'll see some restaurateurs talk about the costs of inventorying a selection of wines; doubly an issue for the rare eatery with a cellar. That's somewhat legit, although it certainly does not account for the full markup. You'll also hear some talk about the cost of training staff, service and glassware, yada yada.
No, the numbers don't add up, IF you are a canny consumer who knows his wine and knows what you pay for it to drink it at home.
But it's the way the system works, and while it's not something that gets talked about often (what business do you know of where the entrepreneur does chat casually about his business plan?), it's no secret, either. It's standard across the industry, and frankly, it's the primary reason why most restaurateurs fight efforts to bring BYO legislation in states that don't have it.
Most diners don't care, and that includes wine geeks. I've made my own peace with it, and my personal strategy for dining out with wine is to favor places with decent lists that include good QPR choices. I'm willing, if not happy, to spend $20 to $30 for a wine that I know retails for $10 to $15. That's the way the system works, and since I enjoy the dining-out experience, I pay the price, knowing that the steak on my plate cost about four times what it would have cost me at the farmers' market, too, and that all the chef really did was sear it, burn it and put it on my plate. <shrug>
You are correct, too, about restaurants buying wholesale. Generally speaking, my rule of thumb for a fair restaurant price - assuming acceptance of the system in general - is that the restaurant pays $7 for a wine that retails at $10, and will sell it for $20. If the restaurant chooses to sell it for $15, they earn a Happy Point from me. If they sell it for $30, I'll go with a beer or iced tea.
But the bottom line is this, Bill: It's not really something to rant about. It's how the restaurant makes its books balance and stays in business for your entertainment. If you choose to dine out at nice restaurants, it's part of the deal. It's a capitalist system, and bless it ... if the restaurant makes money, it stays in business and continues to entertain me.
This would be a great topic for Marsha's monthly Industry Standard column, although come to think about it, since I just wrote it, maybe I ought to make a column out of it myself.
