It's part of the generally poor scripting of III. You're right: there was no real reason for him to have the other Dons as investors-- it would have been even more dangerous to his "legitimate" front than having button men all over the West. As for leaving Zasa out: it was a device for setting up the helicopter attack--except that Zasa had to have arranged the attack well in advance of having been left out of the divvying up of the casino assets.