I don't ride with headphones, but I don't see the problem with it. No one complains about deaf people or people with poor hearing being able to ride safely. They advertise cars on the basis of being silent inside quite often and no one complains about that. You're just as likely (probably moreso) to get seriously injured in a car accident as on a bike. Motorcyclists regularly wear ear plugs while riding, and even if they didn't, you can't hear anything but wind and engine noise on a motorcycle anyway.