Unable to symlink /Users in OS X El Capitan
System Integrity Protection doesn't prevent you creating sym links in / because the root of the file system is not protected!

But the root of the OS X file system already contains an invisible volume/mount point home created by /etc/auto_master (and /etc/auto_home). Your intension to create an equally named sym link there conflicts with this and is therefore forbidden.

Network users who login to the local machine will have their home directories mounted in /home according to the details in /etc/auto_home.