I was under the impression that the Araucana, is the official, rumpless and tufted bird that lays blue eggs. The Ameracauna is the same as the Easter Egger, it is a mix of the Aracauna. I believe the Aracauna has a lethal gene? if I remember correctly.
I'm not sure why they do it, certainly if you are in the business of selling chicken breeds, you would know, but here is what MM says about their birds.
"The "Easter Egg Chicken", This unusual breed gets its name from the Indian tribe of Chile where they were first discovered. Araucanas lay beautiful colored eggs of blue-green shades from turquoise to deep olive. These natural Easter Eggs will amaze your friends and make a great "show and tell" project for school. Adults are of medium size with pea combs and our breeding stock is selected for their ability to produce colored eggs. They exhibit a wonderful combination of colors and color patterns and 10 or 20 of these birds make an absolutely beautiful laying flock that is extremely hardy and will be the talk of the town. Baby chicks come in all colors, plain and fancy, just like the adults. This is a unique breed and great fun to have when the colored eggs start coming. Our Araucanas are recommended for egg laying color and ability and not for exhibition."
Their drawing is of an Ameracauna, and they actually say it lays other than just blue eggs and that it is called, EE. So, it is not an Araucana.
I got two Americanas, Ameracaunas, from the Tractor Supply. Two of the most beautiful birds I have had. My Dh faves. They have beautiful coloring and pattern, and each one is unique. They lay blueish green to olive green, but mostly green eggs.
You are correct that the Araucana is the rumpless true breed that lays blue eggs. The Ameraucanas were bred in the USA from the Aruacanas being brought over from Chile, but have muffs, beards, & rumps. The true breed of Ameraucana lays blue eggs. The EE is a mutt cross of either Araucana or Ameraucana. I think sometime the mutt EE can be spelled Americana, with an I. The hatcheries are very deceiving. It can be very confusing. Boy, I'm confused just reading this. LOL!!!! Hope this helps!