This happens because 2D is using Metacity. Simple fix though...
Open Unity 2D's session settings:
gksudo gedit /usr/share/gnome-session/sessions/ubuntu-2d.session
Change metacity
[GNOME Session]
Name=Ubuntu 2D
RequiredComponents=gnome-settings-daemon;
RequiredProviders=windowmanager;panel;launcher;
DefaultProvider-windowmanager=metacity
DefaultProvider-panel=unity-2d-panel
DefaultProvider-launcher=unity-2d-launcher
DesktopName=Unity
To compiz
[GNOME Session]
Name=Ubuntu 2D
RequiredComponents=gnome-settings-daemon;
RequiredProviders=windowmanager;panel;launcher;
DefaultProvider-windowmanager=compiz
DefaultProvider-panel=unity-2d-panel
DefaultProvider-launcher=unity-2d-launcher
DesktopName=Unity
Save file and exit gedit
While still in terminal:
sudo restart lightdm
Log back in and have fun. Hope this helps :)