Michael shared the casino profits with the other Dons because they were investors.
In the Atlantic City meeting scene,he states that each Don would receive money commensurate with the amount of their original investment,plus the length of time that they had been in.
One Don even asks another "how much did you invest" to which the Don replied "I don't remember"
Since one of the men gets a payout of 50 million dollars.and assuming the others are either above or below that amount,I would imagine that there was some serious money in the casino business.
It should be noted that Michael tells the Dons that their business is now finished,which I believe is because he wants Immobiliare (as he tells Altobelli) to be completely legitimate.