since a non-orthodox Jew cannot serve a (non-mevushal) kosher wine to an Orthodox Jew at the table, effectively precluding a shared dining experience under most circumstances
Not true, as the Jewish person you are with can open and pour the wine,allowing the shared dining experience (assuming kosher food as well- otherwise, it's just a shared drinking experience). I don't find your nor Peter May's arguments compelling. On the other hand, there are plenty of compelling arguments to be had. For one, the wines don't consistently approach the qpr of other wines in their price ranges. There are plenty of exceptions, but as a general statement, it's true. For another, they are not generally available--certainly the best are not generally available, so why even worry about them as a group. They are typically out of sight, therefore out of mind. Another argument would be not wishing to take a chance, given the historical bad reviews that kosher wines have garnered.
At a tasting, however, if one were offered an unfamiliar kosher wine, would one be hesitant to try it, knowing that he could simply pour it out and spit if he did not enjoy it? My personal view is that anyone doing so would be an idiot if the reason for acting in that manner were simply because his feelings were hurt due to nonJews not being allowed to handle and manipulate the product (note that there could be what I consider more valid reasons for choosing not to taste the wine, including having other wines or wineries of higher priority which the taster wishes to taste, and not having the time nor palate to do both).
All wineries all have the same rules. Those authorized to do so may handle and manipulate the wine, those unauthorized to do so may not. No winemaker wants an unauthorized person to open or close a valve or turn on or off a pump, because it might result in loss of product or loss of equipment. Typically, only employees may do so, and even then, typically only winemaking staff. A winery's criteria for choosing its employees is its own business, as long as it is legal.