From my experience using a Bermuda-issued debit card in the US, it had a V/MC stamp so the card could be run as a credit card transaction, so no PIN needed. I was never asked if it was a debit or credit card, maybe a debit transaction couldn't "talk" to the foreign bank for authorization. Your Wells Fargo US debit card could be recognized as a US debit card, so it asked for PIN. It also probably could have been run as a V/MC transaction, but a shop-owning friend told me he prefers to run it as a debit to avoid the V/MC interchange fees.