Almost certainly incorrect, but here's my fun answer:
Mexico City - travellers are warned NEVER to hail a cab unless they want to be taken somewhere unknown and robbed/raped. When I visited Mexico City, I rather walked for 3 hours than take a cab!
Copenhagen - my first ever experience of native Danes in Denmark was getting a cab from the airport in Copenhagen. I've never heard the words "F***ing" and "bull***t" so many times. Clearly it's a dangerous place for getting a cab.
Bogota - it's in Colombia, so getting a cab is probably dangerous.
Ankara and Kabul - not sure I'd want to hail a cab in either of those either.
So, they all have patchy cabbies.