Well one could say that all those mafiosi keep the opportunistic criminals out of the neighborhood and make it safe for the law-abiding businessman and resident

Well one could say that all those mafiosi keep the opportunistic criminals out of the neighborhood and make it safe for the law-abiding businessman and resident. So in essence yes, they built the society by providing a predictable and stable order. Follow the rules and you'll be fine and will be safe from arbitrary crime. A form of societal curation, one might say. And they need paid to keep maintaining this service, and 30% doesn't seem too high does it? For the benefits they provide? Just don't try to fiddle your accounts to hide any other income you have, or try any other tricks; you might find your contract terminated.