Accidentally accepted dynamic currency conversion at a terminal in Rome and lost 4% on the transaction, how is this still legal

Was at a restaurant in Rome last week, card reader asked if I wanted to pay in USD or EUR, picked USD thinking it would be simpler to track my spending back home.
Got home and checked the actual exchange rate I received versus the mid market rate that day. 4% worse. On a $200 dinner that's $8 gone for literally pressing the wrong button on a screen.
The terminal made USD look like the helpful option, familiar currency, no surprises, easy to understand. What it didn't mention was that by choosing my home currency I was handing the merchant's payment processor a 4% margin on the conversion.
Looked into it more after and apparently DCC margins can run anywhere from 3% to 12% depending on the processor. The merchant often gets a cut too which is why some terminals push it aggressively or make the local currency option harder to find.
Been using credit cards internationally for years and knew vaguely that DCC was bad but never understood how bad until I ran the actual numbers.
Always always always choose local currency at the terminal, no exceptions, regardless of how confusing it feels in the moment
Has anyone found a clean way to avoid this entirely or is eternal vigilance at every terminal just the permanent answer