When you absolutely, positively need to be faster than everyone else, you go for the V12. And if you want to do it in style, you probably went to AMG. Mercedes has apparently put two and two together by handing off development of all their V12s to their performance division.
In all truth, V12 development probably should’ve belonged to AMG already. They’re pretty much the definition of a niche part. Outside of S-Class, the only place you’re going to find a V12 is in the AMG lineup anyway. Even though Mercedes only uses the V12 in a few cars, they also sell them to other manufacturers, like Pagani. Both the Huayra and the Zonda rely on a Mercedes power plant for motivation.
Now that production is sole responsibility of AMG, there’s no reason for them to focus on things like “economy” and “fuel consumption.” I don’t imagine that was ever really a consideration in the first place, but I’m still curious to see what AMG does with it.