They essentially just don't have the hookups for all the emissions-related equipment like the factory ones do. So although you would still have the equipment on your car, you'de have no where to hook them up if need be - therefore your vehicle will probably not pass the sniffer test if it ever came to that.
87GT is correct but didn't quite put it in the right words. The emmisions heads for our cars have small holes on the tops of the exaust ports which lead to the very center of the head where they feed exaust gasses to the middle of the intake which routes the gasses to the EGR plate where the EGR valve releases the gasses back into the intake durring certian conditions. There is also a port in the back of the head where the air pump/smog pump pushes air into the head at certian conditions. A non-emmisions head will not have these ports. With a good converter, you should still be able to pass, but you'll burn the converter up pretty quickly.