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

Agreed. They should be in some sort of carpool lane maybe but that lane should not be the same as a true high occupants vehicle like a shuttle or bus.

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But why even give them the carpool lane?

The driver isn't an extra person who's benefiting from being transported from point A to point B, they're there purely to help move the passenger. Taxis are not carpooling. You _can_ use them that way, but you can also use regular cars that way.