When I visited Paris, around 7 years ago, it was by far 2-stroke exhaust that was the problem. My Parisian coworkers, who also rode motorbikes, explained it was because the licensing and vehicle taxes are tiered by displacement. So everyone gets a small displacement 2-stroke, since a 2-stroke makes power equal to a 4-stroke double the displacement.

I haven't checked to see if this has changed since then.