I expect the latter. Taxi drivers can pretend to drive around with customers (I see a lot of them driving around empty) and at the end of the day deposit cash at the bank pretending the cash is from clients. While in fact it is drugs money coming from a gigantic pile of garbage bags with ‘useless' cash.
Also you can see tourists getting in a taxi for just a few hundred meters. They bought a little envelope or some pills while bureaucratically they paid for a ride!