What is it with Cabbies?

In my ignorance I thought the rule (the law???) was that empty cabbies could NOT use the bus lane, only cabbies with paying passengers could. Nevertheless I see tons of passenger-free cabbies on bus lanes... am I wrong? is that kosher?

I've never heard this interpretation before.

My understanding is that if it is a taxi ( not mini cab) it is generally allowed in most bus lanes irrespective of fare paying passengers on board or not