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?

in Lisbon it’s only official city taxis that are allowed, uber and bolt aren’t

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Which implies that it's because official city taxis pay some kind of tax, and so this is an obfuscated way of paying for priority. So... why not just add a pay-for-priority-passes feature directly, and let people with their own cars also benefit from it if they choose to pay the same tax?

People who use taxis don't consume parking spaces, which are also a scarce city resource, and they get priority in traffic a reward.

Arguably, the same rule should be applied to Uber and such, perhaps with the same taxation as well.