You get to kee your free spins winnings as they are your casino bonuses. Free spins are not demoed bonuses but exclusive offers you get in the form of deposit casino or casino no deposit bonuses. If you play your slot games with your free spins and get lucky to win, you get to keep your free spins winnings. There might be some clause for keeping your winning, as some casinos will demand some requirements. Overall, you get to keep your winnings and bonuses.