Funny, In addition to my Guzzi, I have an old Harley with VERY LOUD pipes. I've logged well over 230,000 miles on this bike. I've been all over New England, and have toured Quebec, and Nova Scotia many times. Not to mention 3-4 rides from Boston to Key West. I have NEVER been pulled over for my loud pipes, ans these things are loud! It hasn't even been an issue crossing the border into Canada where it seems to me many riders keep their pipes stock.
I think the reason for this is that I'm not a total jerk about it. If I'm riding through a residential or downtown area I keep the rpms very low and just putt along on torque. I don't sit at a red light and wack the throttle. I hate it when people do that. 'Hey buddy ya got an idle problem!!!" I think that if you are going to ride any bike with loud pipes, if you are respectful of where you are, you can usually slide by without incident.