It's for ripping off the unwary. They then lunch their engines, and the manufacturer in that circumstance won't honor the warranty, so said unwary get to buy another engine too. In many parts of the country, including here, convenience and grocery stores sell off-brand non-detergent "API SA" or "SB" SAE 30 for a little less than they would normal SL/SM oil. All the oil is sold at ripoff prices using the philosophy, "If they need it that bad, we'll stick it to 'em," so the uninformed or those in a hurry grab the cheapest oil. Which happens to be the ND.
Some compressors and other shop and industrial equipment call for ND SAE 30 or other ND weight. Years ago, some mechanics used to advise customers to run a quart of ND 30 in a full tank of gas for breaking in new engines. Wouldn't try that these days in most cars, as this would probably clog fuel injectors.
I have long advocated banning non-detergent oils sold as motor oil from store shelves. Only cars built before World War II specified and would still use ND. California and some other states have instituted stringent labeling requirements on these oils to alert the consumer not to use it in a newer car, but out here these warnings do not appear. There is no reason for this stuff to be in regular stores because of the potential damage it can cause. As a special-order oil for antique cars, this would be all right, but not to sell to some naive young adult topping off his newer car.
This topic has come up before on other threads. On ND oil, some of the other BITOGers believe that "let the buyer beware" should hold at all costs, free market yadda yadda, but I vehemently disagree. Some of the same people would probably also allow infants' elixirs containing opium still to be sold over the counter as was done 100+ years ago. And they would probably still allow sales of fresh bagged spinach leaves containing E. coli bacteria (in the news as we speak, and the subject of a big recall) without warnings to the consumer as well. Hey, caveat emptor!