And in that case the CAPTCHA would still work, as the vehicles are of the same type and have clear branding.

Is 'identify all the similar cars?' a valid capcha though? Surely knowing that something is a taxi is the important part of the test, and a good capcha would through in a branded car that isn't a taxi, which non-Americans are far more likely to fail.

Remember that the goal here is to test that a request isn't coming from a computer, and 'identify all the similar things in these images' is really easy for a computer.