Motorcycles 'pollute' (particulates and other very harmful products) more than cars, but 'emit' less (carbon dioxide and other exhaust products).

Many larger, more powerful motorcycles actually emit more carbon dioxide than modern hybrid sedans.

My R1 had a cat, and that was way back in 2004. This definitely isn't some kind of absolute truth, CA-legal sportbikes have required cats and EFI for a while now. I'd have to check to see if there's a displacement threshold that excepts the small fry though.