Actually, just like cars, all motorcycles must meet strict noise level requirements set by the EPA, and have since the early 70s. The bikes ship from the factory with compliant exhausts (if you look carefully, the muffler cans and tailpipes are stamped with a message to this effect). Many riders remove these exhausts and replace them with aftermarket units. For sport riders this is done to increase the bike's power, since the stock exhausts are too restrictive. For cruisers ("hogs", as the author puts it) this is indeed mostly cultural. Many riders swap the stock exhaust back on for the annual inspection. It's very hard for police to enforce since the sound meter equipment is tricky to use.