Having it printed somewhere on their site is way short of being good enough: A real warning, about something that is so unusual and discriminatory, is a warning that appears over the button, just as you are about to press buy... Better yet, the system should be designed to make completing the purchase impossible, as it is done everywhere else, like Ebay...
Other large retailers in the US are also restrictive, particularly hardware stores for some reason, but at least they simply prevent you from completing the purchase during the shopping process, so you are not left with your CC funds tied up for a week for a dud sale. Taking your entire purchase money away for a week, and then telling you your transaction will not work, is unacceptable.
Gaston