At least in the UK, there is strict licensing for anyone who wants to take a passenger for commercial purposes. And there are different licences for different types of taxis. If they do not have a licence, then their insurance will be invalidated. The authorities crack down heavily on unlicensed drivers.
To be honest, I would expect this to be the same around much of the rest of the world too.
Evidence? My father runs a cab company, and also the fact you can't go anywhere in London without seeing scaremongering posters warning of the dangers of unlicensed taxis.