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 havent checked that, but I bet, in some countries taxi drivers use bus like without it being officially allowed 😅 But if counties start to allow it officially, it doesn’t make sense to me, the whole point of bus like is that public transport should have advantage over regular cars and traffic, to deliver people in time according to schedule, etc.