I'm pretty sure the issue is access to the rear wheel. Owners must have clear access to fix a flat.This is a good question, I guess it depends just how close the exhaust is to the tire. I know exhausts are commonly routed fairly close to rear tires and/or spare tires without issue so I am guessing automotive engineers already know how close is too close.
I am wondering why the exhaust crosses from the right side to the left. Did they have clearance issues with the swingarm or was it something else?