The Mafia forbade kidnaping to protect its paying customers.

Evidence

It is a fact that in the 1970s in Sicily there were very few kidnapings by comparison with the rest of Italy for the same period. Kidnaping was forbidden mainly because its impact on regular customers was so drastic. When a kidnaping did occur, punishment was merciless. — Diego Gambetta, The Sicilian Mafia The Business of…, 456

Categorization

• XREF: Directly evidences the ‘business of private protection’ thesis — the mafia forbids activities that scare its customers, marrying Gambetta’s own model to this concrete rule.
• ADDENDUM: Connects to the private-protection market mechanics: protection firms suppress the very predators (kidnapers) that create demand they cannot profitably serve.
• QUESTION: Why did the mafia forbid kidnaping rather than monopolize it? Gambetta touches the impact-on-customers mechanic but the alternative-profit logic is only implicit.