Why do ex's always seem to want to be friends?

I have been with my new b f for about 8 months now , known him for about 3 years... his ex and his ex ex decided to become best friends, with all court proceedings etc, they show up together, and are trying to financially bury him. now they both have children with him so at-least they do have one thing in common besides how they got f$%ked and how they can both do the same to him now in a completely different way.... I seen a little baffled when i found out that his last ex and her b f split up.. and now my b f and him seem to want to be friends.. this doesn't sit well wit me.They have only one thing in common and thats how they f@#ked her...can anyone please tell me why i'm thinking this seems wrong?