The reason Xiuling needed Jing Yuan's permission to see Jing Xin is because the secret key to the Crystal Prison is only held by Jing Yuan. No one else can open it. The prison itself exists in a relatively isolated and solitary space, accessible only through a phase transport device.
After all, the prison was designed to detain the most dangerous criminals across the universe, so preventing jailbreaks was the top priority. The security standard was so stringent that even people outside couldn't easily open it.
