Now that school is back in session, I see teenagers from my neighborhood doing this all the time. I suspect that they wear it out of the driveway and around the corner. Once they're out of mom's sight, the helmet goes on the handlebars.

When I'm feeling snarky I'll ride up alongside and say "Hey! That's a really cool hemet! But, uh, I don't want to embarrass you or anything. You know it won't work unless you actually put it on your head, right?"