Power of the presidency, obviously.
At work last night, I read an article about the media restrictions to this. The article said that because the remains were "co-mingled" into different caskets, the press could not be there. The article said that not only do the caskets contain body parts of the Americans, but also those of the Afghanis. The forensics labs at Dover will be sorting through each casket to identify each part, so that each family will receive only their family member, and the Afghanis will be returned to Afghanistan.
A service member's family must sign off on allowing pictures, video, press, etc, to be done when their loved one comes into Dover (or anywhere).
According to the article, because of the co-mingling, not all families had signed off. It also said that the government could not sign permission.
Obviously that doesn't apply to Obama's White House releasing pictures.
http://politics.blogs.foxnews.com/2011/08/09/remains-service-members-lost-afghanistan-crash-return-dover