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?

Bus lanes are not max utilized in 24 hrs by allowing only buses? Taxi = ride sharing just as bus but in smaller scale. Could be the justification. I remember in North America there were specific lanes for ride sharing cars

Building @nanograph moding /singapore

https://github.com/nanobrowser/nanobrowser

In The Netherlands it's dependant on the local regulation. But a taxi always has to give way to a bus when it's entering the lane again after leaving a busstop.