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?

Because they are seen as public transport. Technically they are only supposed to do so when working. Personally I prefer to have them in the taxi lanes as they are among the least predictable drivers.

But taxis don't have any of the key properties of public transport that usually motivate actively subsidizing its adoption?

* environmental friendliness
* low contribution to congestion
* high-fixed-cost cost curve

"You're annoying so you get to use the fast lane" feels like good short-term move but bad long-term :D

Taxis were public transport before environmental friendliness was a thing. From a policy point of view, taxis are for people in a hurry or with special needs.