They're supposed to enter "00000" or "99999" as instructed by their card providers. They also can go to Walmart

.
The real reason why your C&S is working fine in Europe now is due to a similar Visa and Mc Requirement where they're working to update those European machines to accept C&S cards, just as how we have to take their Chip & PIN cards here. Example: At Walmart if you were to insert a Chip & PIN card, the machine will prompt for a PIN, because it's on the CVM, or Cardholder Verification Method, for that card. Visa and MC are doubling down on correct CVM enforcement, so theoretically a machine will always listen to the CVMs on the card, so a US CARD that is C&S, should work everywhere, and vice versa. This explains the trouble of that airport shop mentioned earlier, the machine probably asks for a PIN, but the lame clerks instead of handing the machine to the customer, puts up a handwritten sign banning Chip & PIN cards, which is against the merchant agreement.
If merchants listened to their agreements more, than there shouldn't be any issues regarding using "a UK card" in the US, etc. solution? Report these ******* merchants who must not like money. It is in their agreement to take any card with the Visa or Mc Logo, regardless of stripe, Chip and PIN, or Chip & Sig. Sadly nothing in there for NFC yet, I hope they require NFC SOON in the US, like they do Chip and PIN/Sig.