So there is a difference between interaction source, and money source.
That I agree with, except in the situation of a new casino, where they will have to put together a house bankroll to manage the variance of wins and losses until the player pool grows to a suitable size.
I`m not sure how physical casinos are born, they can raise money by stock market, fundraiser, or bank loan to have the initial capital for house->player games.
However in bitcoin it's easier, many bitcoin casinos like Satoshidice.com, offer investments in their bankroll, so investors can invest directly in them, and gain a % share from the house profit.