After all, if there's a dispute over a listing, they need to be able to see it as it was at the time of the sale - which means storing copies of all the images.
It would give them control over page-load times and avoid leaking data about customers' browsing habits. And both sites have obviously demonstrated the ability to host (and pay for the hosting of) user-submitted images, as they have much better revenue sources than Photobucket.
(I agree it sucks about forum posts)