As mentioned above, the first time one sees the message, you might think hitting "decline" would cancel out the entire transaction. No mention of 'paying in own currency' as in the case of buying from a merchant.
Perhaps this is a thing with US-issued cards only ? Because my European bank card (Mastercard) never prompts for DCC when paying or withdrawing cash in a foreign currency- I'm sure this would run afoul of EU consumer protection rules !