re: "...Incidentally I was checking the german language MB website and they specify the braked towing capacity as 3100 kg. Why I don't know. Perhaps local regulation?...."

Yes, it is strange. I noticed this same thing when I was comparing M-class towing ratings (in my other post, "towing capacities" next down on the list).
At the parent MB website, M-classes (both 320 and 430 engines) are rated for about 7,400 lbs (can't remember exactly), while in the USA, the official brochures limit towing to only 5,000 lbs. Clearly, this has to do more with national differences than differences in the vehicles. National regulations could be the factor, as you noted -- as well as corporate concerns about such things as legal liability, the USA being far more litigious and therefore justifying far more conservative capacities.

Perhaps we'll have more indication when we finally see the "official" USA brochures on the GWagen, for comparison with this website?

Safe sailing.