It's to protect you from yourself (and malicious software).
Anyway, where are you trying to move the contents?
I know there's one in /usr somewhere (which you'd need to copy as root), but you could instead move the contents to ~/.themes
PC: Manjaro/LM 13 x64 XFCE, Kernel 3.11.1-6/3.8.0-32-generic, Nvidia GTX650, i7 3770K, 16GB RAM, Asus Xonar DX
Laptop: Lenovo G560, LM 13 x64 XFCE, Kernel 3.11.1, i3, 6GB RAMMy HTML5 login themes + screenshots