Hi Garry,
this behavoir comes from the gateway, not from the binding. Have a look at the tradfri app. If you switch on a bulb, the app reports it as online. But if you turn it off with the main switch, it stays online. Same behavoir as in the binding. 
I’m searching a solution since i’ve got my first Tradfri bulb.
All my bulbs are installed in sockets which are switched through wall switches. Cause i’m living in a rental object i’m not allowed and don’t want to change the wall switches.
It would be great if the status of the bulbs is reported so i could use it as trigger for some rules. It would be even greater if i could change brightness and color of a bulb even it is switched off, and the bulb pulls this values at the moment i switch them on.
But i think this is something Ikea has to implement. 