Well, it's true that a cab would be a horrible environment for secure POS...
Lots of well-off businesspeople swiping credit and debit cards, lots of partiers doing the same, and any crook can get into the back of the cab, and can probably fiddle with the POS terminal under the guise of "Just a sec. I'm having trouble getting this thing to take my card."
Plus cab companies are well-known for being pretty cheap, and that includes IT security.