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See the discussion in another thread about the confusion caused by using nonstandard abbreviations.
Despite that, I get the gist of your post. Coming from a background of other species in which purebred animals are registered and pedigreed, it is an alien concept to grasp when it's allowed in "purebred" poultry. Because the unit of importance in breeding poultry is the flock, not the pair, the rules are somewhat different. In exhibition poultry, purity is deemed by how well an individual measures up to the Standard. If it LOOKS like a purebred, it is considered to be. Seldom does one individual bird have a great influence on an entire breed, like what might happen in dogs, horses or other animals. Individual poultry seldom remain showable for more than a year or two, leaving the skill of the breeder to be recognized in the long term by how well he can continue to breed similar winniers of consistent type. The introduction of other breeds will soon lead to a breakdown in proper type IF the breeder does not know how to select properly for correct type, so one individual bird even if mixed, yet looking like a purebred bird, will not make a breeder successful in the long term. Their long term success will be proven in the consistency of type in his strain, so ultimately that is what is important, not a piece of paper which proves purity. As I hope that you know, registration and a pedigree does not guarantee quality. A poultry breeder has to prove the quality of his birds at every show in the flesh, not just on paper, so in many ways it's a more practical approach than registration.
The other issue is inbreeding. Like it or not, every purebred animal has some degree of inbreeding. That is not always a bad thing, but done improperly, it can lead to disaster. There are many instances of a well known sire in dogs, horses and other animals, which had some good traits and was used extensively in the breed, only to find out later that he carried an undesireable recessive trait. In some cases these sires have become so well used before everyone discovered the problem, that in the entire breed it is difficult to find individuals which do not have some relation to him. Issues like this have caused some breeds to be plagued with genetic problems common to the breed. In every dog breed there is a list of problems that occur with some regularity. Outcrossing would help to eliminate these things, which is why they are still problems, because in dogs it is prohibited.
Also in poultry there are many varieties which are recognized which no longer occur in the breeds in which it is allowable. The only way to get these back is to cross it in from another breed, and it is done fairly regularly by some breeders. It could be argued that if a variety in a breed is extinct, it should be removed from the Standards. I don't disagree, but as outcrossing is still allowed for other reasons in existing breeds and varieties, it's kind of hypocritical to say that you can't do it to recreate a variety.
For some good information on why and how this is done properly, see Bantam Chickens by Fred Jeffrey, as well as some of the other older publications by the ABA.