In 2006, Congress passed the Unlawful Internet Gambling Enforcement Act, which made it illegal for wagering businesses to knowingly accept payment in connection with unlawful Internet gambling (though it does not itself make Internet gambling illegal).
Yes, some free slot machine apps offer the chance to win real money. These apps often have in-app purchases or advertisements that generate revenue for the developers, and they might also have a system where you can earn virtual currency or tokens by playing the free slot games.