has been granted the license to redistribute under the same license. So commons would act as a proxy between the original uploader and the reusers of the content, shielding the reusers from the whims of the original uploader.
But wouldn't that make commons more vulnerable by the same argument?
In what respect? Liability for copyright violations? We are taking those pretty seriouly in any case.
It boils down to the question whether commons is
a) just a marketplace, a technological means which only facilitates a transaction between the original uploader and the reuser
b) a third and equal party in the transaction, standing between original uploader and reuser
And I think we are kidding ourselves if we just assume a) is the case. Commons hosts, displays the images. It effectively 'uses' them. We couldn't afford to leave copyright violations on the site in any case. The vulnerability is already there (just as vulnerable as any entity on the internet, ignoring this doesn't make you bulletproof ;-) )