That is what I always thought as well. It turns out it is due to the type of aluminum composition they use I believe. if they spec the right aluminum they can anodize it. I ended up doing that on a large part last year where machining was just not possible due to the size and the voids. We would have been machining away 90% of the material over a part that was rather large… after a bit of back and forth they figured out how to cast, post machine, and anodize it.