The Chocolate color can be registered...it just isn't correct, and shouldn't be bred. The AKC is not denying that the color exists -- they will register it.
It simply denotes poor pigmentation.
You will likely find "tons of" all over the internet, crappy breeders breeding the color on purpose out of AKC registered dogs. These breeders are ignoring health, temperament, and type in favor of producing a "rare" color that they can charge more for. If color is more important to you than health and temperament, buy from one of these people. If, however, getting a well-bred, healthy Yorkie is more important, buy from a YTCA-member breeder. Yorkshire Terrier Club of America