Where the BTC they give comes from? They buy it all to make the giveaway and for the gambling purpose?
Faucets main source of income are advertisements. They usually need to give away something initially before getting paid.
Casinos earn from players losses. Casino faucets are just a way to advertise or to allow you to play around with small amounts first before you deposit. They usually come with a withdrawal limit that is very unlikely to reach from the faucet amount. Casino need to hold funds the players bet against. This amount is usually significantly larger than the initial amount needed for a faucet.