Yup. I doubt banks actually want people that are too "ethical" as it's a dirty business. They surely want to portray this "hard honest work" image but most know what's up.
Yeah, ethics isn't much to do with this. Its a matter of internal politics. Although, maybe there was something more serious (more embarassing for the employer) and the sandwich issue was a good black-and-white basis for suspension. But either way it puts in mind of Machiavelli (I think it was him), who said you must always kill someone who has done you a minor hurt. Obviously, if he committed some great injury you would have to kill him but why so for the minor one? Because if you don't, and you only harm him in an equally minor way, he is now injured and is now walking around bearing you a grudge. In the corridors of power in 16th century Italy, nobody wanted that, and I suspect its the same in high level banking.