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?

I'm guessing they are more environment-friendly since a single taxi will, in a given day, provide transport to multiple users who might otherwise use their own (multiple) cars. If this is true (not sure if it is; those multiple drivers would probably drive only a tiny fraction of the time, while the taxi is driving ALL the time, regardless of whether it carries passengers or not)... so, if that assumption of taxis being more environment-friendly is true, then applying policies that encourage using such service seem rational (reserved lanes = faster trips = more demand of taxis = less cars in the road). But in the end, who knows... So many dumb policies out there