Someone humming down the A1 at 140+ may well have occasion to use their brakes hard repeatedly as the occasional Ford Fiesta pulls out to pass.
You have a lot of thermal shock asking a stone-cold rotor to dissipate 100mph in a hurry, and the floating rotor design is less likely to warp/crack in these conditions.
I suspect the reasons Americans don't get them are:
1) A perception that Americans don't have occasion to use their cars in this manner.
2) Experience shows that Americans tend to run weeping to their dealers at the slightest noises coming from their brakes; floating rotors tend to be noisier.
3) The rotor-to-hat interface is a wear point, and BMW warrants the cars longer in the US.
So, BMW puts (a) quieter (b) cheaper and (c) less likely to cause warranty replacement (if not used hard enough to warp/crack them) one-piece rotors on the US cars to save themselves grief.
You have a lot of thermal shock asking a stone-cold rotor to dissipate 100mph in a hurry, and the floating rotor design is less likely to warp/crack in these conditions.
I suspect the reasons Americans don't get them are:
1) A perception that Americans don't have occasion to use their cars in this manner.
2) Experience shows that Americans tend to run weeping to their dealers at the slightest noises coming from their brakes; floating rotors tend to be noisier.
3) The rotor-to-hat interface is a wear point, and BMW warrants the cars longer in the US.
So, BMW puts (a) quieter (b) cheaper and (c) less likely to cause warranty replacement (if not used hard enough to warp/crack them) one-piece rotors on the US cars to save themselves grief.