Yes, it is true that the different species of carpets have ranges that overlap and that there is some amount of interbreeding going on. However, there are still places in those ranges that you can aquire pure specimens.
While you may enjoy an intergrade as a pet, there are others out there that would rather have the real thing. It is still possible to find pure coastals, jungles, and diamonds in the States regardless of the babble that most keyboard jockeys on forums spout. If you can find a reputable breeder that has good lineage info on his animals you can rest assured that it is pure. There was several animals smuggled into the States decades ago over a small timespan that introduced pure blood to the States. There is also some pure animals that are being produced in Europe that are being legally imported into the States as we speak that will form the basis for new blood with lineage info and locality data.