They claim they didn't add support because "the GIF format is not widely used for photos"[1]. But it is very obvious they did this to promote the usage of IE. I wouldn't be surprised if they purposely removed already written code just to cripple the Windows Photo Viewer.
I would like to shame Microsoft engineers who frequent HN into fixing this.
[1] http://social.technet.microsoft.com/Forums/en/w7itprogeneral/thread/b0e84027-9753-46cb-94b8-b099c0a60e1d