Why does the US version need to have a more powerful engine?
Of course, it helps that most low-powered cars sold in the EU will have manual trans, where all US MBs are autos...
Also, US buyers will run the AC a lot more, and AC eats a lot of power...potentially as much as 15-20HP... that's a lot more noticeable with a tiny engine.
The US is a mature product.
High end brands don't sell sub-lowend products.