Steam doesn't really care about the 'elsewhere' so long as they're not liable for it.
At one point the Domino's app was being used to facilitate credit card fraud: https://www.nytimes.com/2014/12/06/nyregion/pizza-orders-rev...
At one point the Domino's app was being used to facilitate credit card fraud: https://www.nytimes.com/2014/12/06/nyregion/pizza-orders-rev...
I never saw that original Domino's article but I can vouch for its complete validity and accuracy. I'm sure carders are still checking CCs on Dominos too. CC checkers are a hot commodity and carders are creative people. RedBox is another good example like Domino's.
Anecdote: In 2008, Obama's donation gateway was also heavily used. I don't believe that was nearly as reported, though.