Sauce for the unvaccinated goose is sauce for the vaccinated gander. If you're fine with the unvaccinated being banned from stuff (saying, for instance, "it's a free society but choices have consequences") then why is this different? The potential passengers chose to be vaccinated, they can deal with the consequences.
On the other hand maybe, like me, you think medical information should remain private and discrimination based on vaccine status should be illegal. In which case the taxi company are of course in the wrong.