They understood the licensing just fine, they also understand covering their ass explicitly. They don't feel like being left to your whims should you choose to change license to attempt to extort money out of them after they start a large campaign.
I'm not a lawyer but this is wrong, if you've released material under a license in most legal jurisdictions that's binding, releasing it under a different license does not invalidate the terms of the previous one.
How do you prove that the license really was granted if I take the website down? Convince a court that archive.org or a printed screenshot is evidence? If I falsely claimed to own the rights to the images, and you get sued by the true owner, can you show you acted in good faith and did enough to verify the license? If you have a contract with me and I lied, you probably would have better chances of recovering your damages from me.
It would be interesting to see how many companies would pay a small fee for this if you asked.
Fair enough. Naive question... Does the Wikimedia Commons (and other established sites) not address this? Licensees of CC material will find content and license change histories, archiving, and (thus) audibility if a dispute arises. i.e. claims like "this was the license on date X" become verifiable