I don't really understand that... In any other species, even if the color is not correct, or can not be registered, it's still considered that breed if the parents are purebred. Why is it different for ameraucanas?
That is actually not correct. In quarter horses, it used to be if a foal has too much white, then it was a "crop out" and could not be registered as a quarter horse with the AQHA. It could, however, be registered as a paint with APHA. (That rule has changed in the last few years, which imho, was a mistake) Even though both parents are pure-bred, registered quarter horses, that foal is not considered a quarter horse.
With dogs, if a pup is born mis-marked, the AKC will allow the breeder to still register it (because they don't require color pics with registration), but a reputable breeder would not give full registration on a puppy that is mis-marked or is colored in a way that doesn't meet the written standard, because you just don't know what it will produce.
It takes more than just saying this animal came from these two like animals for any species or breed to be called a pure-bred. First - does it look like the standard says it should look? So, an Ameraucana should have slate or black legs (depending on the variety), pea comb, beards and muffs, and be colored in one of the 8 approved varieties. Second - will it produce to type? In other words, will it produce within the Standard of Perfection? A cross between a blue wheaten and a blue may look like either parent, but the genetics of the offspring are going to be all over the map.
Keep in mind that a Standard of Perfection is not written to describe a living animal (there aren't any perfect ones), but as a goal for a knowledgeable and earnest breeder to try to achieve. Throwing a jumbled bag of genetics into the mix is not going to help any breeder reach that Standard of Perfection any sooner.