I think it's because they think (don't know how often this is with a good reason though) that most mafiosi have killed at least somebody during their career, so if they aren't able to prove it, then they are going to make them pay an excessively harsh price for minor crimes, so it's as if they were doing time for murder. Just like with the Al Capone stereotype.
Willie Marfeo to Henry Tameleo:
1) "You people want a loaf of bread and you throw the crumbs back. Well, fuck you. I ain't closing down."
2) "Get out of here, old man. Go tell Raymond to go shit in his hat. We're not giving you anything."
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