That's it in a nutshell. Bonasera sues. Vito and sons don't.

Vito was sophisticated enough to understand that "a lawyer with his briefcase can steal more than a thousand men with guns." His legitimate facade--his ability to accomplish things that benefited him and the people who needed him--came from manipulating the politico/judiciary system and exploiting the weaknesses and greed of judges, politicians and cops. A man who could deliver things to constituents "through the system" would wield awesome power and assume mythic proportions.

At the same time, as SC noted, the Corleones would never use the same system to achieve "justice" for themselves in urgent, personal and life-threatening matters. Why should they, when they had other means at their disposal. In fact, a great deal of their strength rested on the fear factor: their ability to go beyond the system and use violence to achieve quick, definitive solutions. The combination was what made them strong. Connie's wedding provides us with splendid examples of both sides. Nazorine wants Vito's "legitimate" political clout to keep his future son in law in America. Bonasera, after trying and failing to get justice through the courts, wants Vito's violence. Bravo!