There are a number of states that have laws making it illegal to buy wine over the internet. These states all say that these laws are in place to stop kids from buying wine. However, the states that do allow wine sales have other restrictions in place to prevent kids from doing the purchasing - such as verifying the age and requiring an adult signature. Sure there are always going to be a few that trick the system, but most states that allow wine shipments report no reported problems with sales to minors. The real reason, of course, is to protect brick-and-mortar wine sellers who are afraid of internet competition. Now, the FTC has come out with a report saying that these restrictions are costing consumers billions, and they could save up to 21% if the restrictions were lifted. This report is the result of hearings the FTC held last year on the subject, wondering why people couldn't order wine (and other items like contact lenses). Of course, all of this is going to come down to a constitutional issue concerning the "commerce clause" of the Constitution which discusses interstate shipping laws, so any attempt to force a change in these laws, eventually, is likely to end up before the Supreme Court.
That's an interesting point, but it seems a rather superfluous one. From what I know of the wine market, the real issue are rather draconian measures installed by the States to control the distribution of liquor. As a compromise for repealing prohibition, various states basically make it very difficult and expensive to enter the liquor business, or to conduct it as an open market activitiy it should be. My guess that various liquor control laws cost taxpayers a lot more than whatever "internet costs" of such taxes might be.