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?

a doc and general practitioner by day, a geopolitics junkie and deep onchain comms shitcaster by night and both in /doctorsphere

But for the purposes of congestion management, taxis *are* personal vehicles?

The idea is to have less parking lots, ideally, leaving now space for "walkers". Personal vehicles in true sense are used for a fraction of the day, so if you make it more convenient to reach your destination by other means- you would normally do so.
Increasing parking tolls serves the same purpose.