This complaint is similar to that of the BMW 2006 M5, which didn't have real launch control in the US version, but did in the rest of the world. It was an advertised feature There were threats of a suit but nothing ultimately happened. Ditto here.
My guess is the BMW underestimated the task of getting the new RR and its exhaust system approved by the EPA. I bet they had the hardware done, which met the EU and other regulatory body's standards, and then went to get it approved in the US where it became clear they dropped the ball. They were likely faced with a number of difficult solutions:
1. change the exhaust system for the entire world and somewhat harm its weight and power
2. make a special exhaust system for the US that harmed its weight and power,
3. game the EPA noise certification by throttle and exhaust valve opening in software for specific sections of the testing protocol.
They clearly chose door #3. #1 would have been a costly and delaying option. It also would have reduced the appeal of the new RR across the world. Option 2 would have also been costly and made for two different bikes.
US owners have a means to fix the problem: get an ECU tune, which costs about 3% (or less) of the total cost of the bike. It not only provides performance on par with the rest of the world, but even slightly better. And it has the flexibility to further boost the benefits of a full exhaust system replacement.
Where BMW must tacitly go along is in not being aggressive in denying warranty claims because owner gave it the tune it should have had from the factory and is reviewed/advertised around the world. If they do this, they do open themselves up to lawsuits and even worse, a tarnished reputation as a manufacturer for performance enthusiasts.
BTW, the Brentune transforms this bike. Really. I didn't get my flash complete until I returned from two track days (CSS) on their US-tune 2020 RR. Mine now is way faster and drivable.