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?

Public service vehicles. The more we use the less pollution. Is this not obvious?

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But taxis are cars and have about as many end-user occupants as regular cars, so it seems like they would pollute as much as any other cars?

Some of those lanes are reserved for "green"  vehicles.