Because it doesn't meet the standards. It's the same reason that if two pure breed chickens have offspring that do not meet the standard, then that offspring does not qualify.
Pure breed animals can be created by selecting for certain traits. "If I take my Czech working line GSD to an AKC dog show, no one in the show field would be interested in him."
Exactly. For people who care about breeding, it matters. Your German Shepherd does not meet the standard for the breed.
I think mutts make the best dogs, and maybe the best chickens also. But for breeding standards, it matters.
Here is a description of the Labrador Retriever by the AKC.
Size, Proportion and Substance
Size--The height at the withers for a dog is 22½ to 24½ inches; for a ***** is 21½ to 23½ inches. Any variance greater than ½ inch above or below these heights is a disqualification. Approximate weight of dogs and bitches in working condition: dogs 65 to 80 pounds; bitches 55 to 70 pounds.
Here is a description from the Ameraucana Breeders Club:
http://www.ameraucana.org/faq.html
[COLOR=000080][SIZE=13.5pt]What are the major differences between Ameraucana and Araucana chickens?[/SIZE][/COLOR]
Both breeds lay eggs with shells colored various shades of blue, have pea combs, and should have red earlobes. Beyond that few similarities exist in specimens meeting the requirements of recognized poultry standards. Perhaps 99 percent of chickens sold as Araucanas (or Ameraucanas) by commercial hatcheries are actually mongrels (aka Easter Egg chickens), meeting the requirements of neither breed.
According to the American Poultry Association (APA), the Araucana breed must be rumpless (no tail) and have ear tufts. Ear tufts are clumps of feathers growing from small tabs of skin usually found at or near the region of the ear openings. This feature is unique in the U.S. to the Araucana breed. This trait is nearly always lethal to unhatched chicks when inherited from both parents. Tufted Araucanas, therefore, are always genetically impure, i.e., they don't breed true and will always produce a percentage of "clean-faced" offspring.
The Ameraucana breed, on the other hand, has a tail and sports muffs and beard in the facial area. These characteristics are true-breeding. Other requirements of both breeds may be found in the APA's Standard of Perfection and in the American Bantam Association's (ABA) Bantam Standard.
[COLOR=000080][SIZE=13.5pt]What are Easter Egg chickens?[/SIZE][/COLOR]
The Ameraucana Breeders Club defines an Easter Egg chicken or Easter Egger as any chicken that possesses the blue egg gene, but doesn’t fully meet any breed descriptions as defined in the APA and/or ABA standards. Further, even if a bird meets an Ameraucana standard breed description, but doesn’t meet a variety description or breed true at least 50% of the time it is considered an Easter Egg chicken.