Addison Lee drivers told to defy bus lane law (and to just say SMIDSY)

How come black cabs are allowed to use bus lanes and AddLee/private cars aren't? I can understand why all taxis wouldn't be allowed to drive in the lane (otherwise it would be pretty easy to slap a taxi sticker on your car and drive willy nilly) but why give an exemption to (often equally shit/aggressive) black cabs?

Training and a level of route knowledge.