A more technical response is that their servers block requests to any of their hosted images by limiting which IP's can request it. It's known at 'hotlinking'.
It's to reduce bandwidth use (and associated costs) and, of course, prompt people to buy the service.
I had this issue when someone directly linked to images on another of my concepts, thereby infringing copyright. It also totally skews any useful statistics.
If fun though when you see their website full of 'STOLEN FROM' images instead of what they expect.
