If I had to guess, adding in the aluminum to the roof would add to the costs, and only was done to meet the US requirement for there to be metal support in the roof when you have a sunroof. Prior to that, the sunroof was not able to be offered in the USA. Personally, I'm glad, it made finding one with the options I wanted easier. Depending on the model, a sunroof can cause you to loose as much as nearly 3" of headroom (I had an Audi A6 once, the difference between with and without a sunroof was 2.85"), which I cannot tolerate. The US tends to make a sunroof standard on many cars, so that limits my choices of which ones I can comfortably drive! BMW specifically won't 'delete' a standard option, even though it may be an option in other markets for sale in the USA.
2011 535i x-drive GT, 2014 i3 BEV