The relative lack of DCC in the US may also have to do with "knowledge". Most everyone going to the US knows what the exchange rate should be. Most everyone (and Americans) going elsewhere cannot be said to know the rates and calculations that well.

Same reason you will NEVER (even if allowed) see DCC between USD, Bahamian and Bermudian dollars, and the Panamanian balboa, although they are technically different currencies.

A scam only works on those who don't know enough.