They're doing the survey on the street. What he is has no relevance since bus drivers catch buses as passengers also.Exactly!
As you accurately said,"they can be passengers".Which therefore destroys the punchline. Bus drivers can also be passengers. Or does he pretend he's driving the bus even when he's a passenger? When ,he plays Bus Driver does he also do the sound fx - vroom vroom, ding!, screeeeech ... ?