Because the Obama campaign hasn't repudiated them, and in fact, linked the same image or a copy of the image, that was there. To me, sharing the image is a tacit endorsement of what Kos said.
On the other hand, I've said from the beginning that I thought this was a rogue operator who thought they were helping, that the Obama webmasters thought it was smart to link it to their (at the time) brand new Fight the Smears site without thinking through all the consequences, and they are now stuck with the mess they created. Therefore, they are just ignoring it now and hoping it goes away.
-PJ