After getting what they wanted all along, why chisel Corleone territories?
The agreement that Barzini and the other Dons made at the Commission meeting was limited: Vito agreed to provide political protection for drug trafficking in the East, Barzini and the other Dons agreed not to interfere with Michael's safe return to the US. Nothing else was covered. Why wouldn't Barzini chisel Corleone Territory, especially since Michael and Vito were feigning weakness? If they didn't like it, they could fight back.
