That’s cheating their company (and/or setting up for tax evasion), which in my experience is far more common than cheating the customer with a detour, at least when the customer is local (you can show your localness through verbal cues implying or simply indicating you know the route).

Here in the UK, if you say it's all you have they just reply "I can stop by an ATM"...

Only the other night a Chicago cabbie tried to get my wife to pay with a portable credit card reader not the one installed in the machine. She made a big fuss and he 'discovered' that the built in machine was working after all.