I also found an interesting blog on the topic, Dynamic Currency Conversion: Still A Scam - Jeanne Leblanc | Coach Class, which provides a link to VISA's (USA) FAQ on the issue.
So I checked the VISA Canada version which states,
The bottom line seems to be general agreement that it is a scam and you do have the right to decline, although it would seem that merchants do not make it easy and count on those looking for convenience over cost.VISA Canada wrote:If you do not want to use DCC when making a purchase, then you have the right to refuse the offer and have your transaction billed in the merchant's local currency, which will then use Visa's conversion rate. If you did not agree to DCC, but see it on your bill, then you should ask your issuing bank to contest the charge.