Silence in the library/The forest of the dead

After River Song sacrificed herself in 'Forest of the Dead' All that was left of her was a small amount of residual memory. As with those who died earlier in the episode(s). The only reason River could be saved was using that memory. If River Song was to be transported out, she would be completely lost.

To add to what has already been said, Donna and the 4022 were all alive when they were transported. (Tranported just like on 별, 스타 Trek) There was no place to reassemble their molecules, they were saved on the computer's hard drive. The Doctor could retrieve them and they were jsut as they had been. River's body had died, but the remains of her consciounessness was downloaded into the hard drive where she "lives" a virtual drive. He she was removed her consciousness would die because there is no body for it to go into.
I would have answered this much sooner, but I wanted to rewatch the episode to make sure my memory was correct. (: