I'm a motorcyclist too, and straight piped Harleys make me stabby. I don't believe they're louder on the highway. I believe they're louder whenever they're within earshot of any place where people congregate without earplugs (cafes, restaurants, pedestrian areas), whereupon the scumbag on the seat finds it necessary to go wide-open throttle to show off how terrible their bike sounds. It never fails.
It's not true this helps on the highway. In my car, which has double glazed glass, you can't hear a bit until it's right outside the window.
When I'm walking along with the dog and kids, an open pipe Harley riding past is particularly not safe, because it scares the crap out of them.
One idling Harley is louder than a thousand idling Volkswagen diesels.