Points well taken, Lilo.

I suppose that, despite being forced by the other Dons to protect the drugs business, Vito's political clout may have expanded because he had more money (as intermediary for the other Dons) to dispense to his police/political allies. And, while Vito's agreement to protect the drugs trade wasn't binding on Michael (any more than Vito's pledge not to be the first to break the peace), I can't see Michael walking away from a source of income.

Control of legal gaming in Nevada ca. 1955, when Michael moved the family, was under the state's Tax Commission. Supervision was lax: all they cared about was bringing money into the state. Still, Michael was a relative latecomer to the gaming industry in 1955, and it probably would have been helpful to him to have "good references" from NY politicians when he moved to expand his Nevada holdings.

Also, since Michael was still in control of the olive oil business through Clemenza, he might have wanted to be certain that Clem could count on police/political protection to the same extent in order to guarantee that Clem would be secure (and constantly able to pay tribute to Michael).