That is done because XMM is not compatible with Vista and higher. It needs writing rights for it's own Program Folder. With Vista and higher Program's don't have writing rights to the "Program Files" or "Program Files (x86)" Folder. So it's installed by default to a Folder with writing rights.
You could install it to "Program Files" or "Program Files (x86)" Folder, but you would need to manually change the owner and the access rights for the XMM subfolder, all it's files and subfolders to full access rights. If you don't the Program will produce errors...