It is a commercial thing... No racer would buy an engine saying "basic" or "stock" and no driver (on all levels) with a limited budget will buy high end engines.....
I do run the N21-5T but I do modify them. Out of the box it is already a good engine and after some modding they are as good as any other more expensive engine. The basic engines are made a bit slower through timings, a smaller hole in the crankshaft and missing optical things like a silicone insert.