I think it has to do with the banking system and the banks, as opposed to the merchant. I've tried to use the Visa debit option on my debit card as opposed to my traditional Visa card on a number of websites, and all have been rejected. I was told (and dont quote me on this) that the online merchant system still gets "confused" when trying to process a debit Visa, because a Cdn debit card (with or without Visa) defaults to debit and not credit. Basically you enter your info and the system sees it as debit and not Visa (credit)
However, I've had zero problems using the Visa debit option when buying online from the States.