Batman is very mysterious in the movies. He beats up criminals and then takes off. Nobody sees him during the day or knows who he is. One would expect the waiter to be quite shocked, but the opposite happens = humor.
Stretching the phrase “unfinished business” for the ghost to “unfinished beesness” is the original pun. Although I think “unfinished beesnest” could have worked too…
There is an Orkin commercial playing in the USA right now about hiring a pro to kill bugs. Turns out they got a Tennis Pro to “swat” the bugs.