Continuity mistake: During the show's opening credits for the entire run of the series, when Tony drives up to the NJ Turnpike toll gate, the lane 7 machine spits out a toll ticket, and Tony reaches for it. It then cuts to a close-up as Tony grabs hold of the toll ticket, but this ticket machine is now different; note all the stickers. The close-up was shot at another lane's ticket machine.





Answer: It's a mafia thing. Taking food off another made man's plate in public view like that is comparable to the guy in Casino taking off his boots and putting his smelly feet up on the table.
It is not a "mafia" thing, it's basic manners and plain gross for another person to stick their fingers into a plate of your own food. Same with the feet on the gaming table scene in Casino. It's not a "mafia" thing, it's just rude and not something a person does in public.