Here's a link to some interesting info on San Marzano's. http://www.sanmarzanotomatoes.org
The problem is that calling non-DOP San Marzano's, San Marzano's is that it is misleading. As I got further into pizza, I wanted San Marzano's. Initially it was based on price and I bought less expensive can's of San Marzano's, which I later found, much to my dismay, that they were not truly from the San Marzano region of Italy as I expected they were. Anytime confusion is created in the marketplace, it can become an issue. Those who only look topically into SM's may never try true (by the Italian definition) SM's, and that's an issue IMHO. You can grow a plum/roma tomato and call them SM's if you want, but they are not, which is my opinion about American grown SM's. If the tomato companies weren't trying to acheive sales from the SM name, they would make it very clear that they are not from the SM region instead of making it look like they are. Why do they do it?? For sales, period, and it works.