In multiple European cities now I see taxis being allowed to use the bus lane (while of course regular cars are not).

What's the policy justification for this?

I like it. 

When I need to move fast to e.g., catch a plane, I take a taxi. 

Taxi drivers need to have a license, so they pay back some money to the city for utilizing the bus lane.

Building @kiwi 
Writing @ https://kanfa.macbudkowski.com/

If the goal is to charge for priority access to give a way for people who really need it to get priority and raise revenue, why not decouple the "taxi vs car" aspect from the "paying for priority or not" aspect?

IMO because:
- rich people would just buy this access and use the lanes even when they're not in a rush,
- taxis are shared and utilized most of the time vs. private cars that move from point A to point B and get parked.

I'd also add that taxis are easier to distinguish from other cars than an extra paid for license or sticker might be. Less easy to game