The worst thing the taxi mafia did to me was force me to use cash instead of card a lot of the time... but that was due to the credit card transacion fee mafia hah
In my location (West Coast, USA) local tradespeople will give me a 10% discount for paying with cash. That's more than they'd pay in check or credit card processing fees. I assumed that taxi drivers' preference for cash was for basically the same reason: under-reporting income to tax authorities.