So I think it is because I created one of this websites as admin and he is not allowed to overwrite the settings, right?
Yes, thats right. This is a feature in ISPConfig to protect the website. Sites that were created by a dmin for a client can only be changed by the admin while sites that had been created by the client himself can be changed by the client.
You can see the difference in permissions in the sys_perm_group field of the web_domain table. 'riud' indicates that the client can save the changes in the form while 'ru' indicates that he cant save the changes. The variable 'sys_perm_group' is available in the template if I remember correctly, so it should be possible to either show a a message in the template that its read only or to disable the fields with jquery. This message or field disabling should only happen to clients, so it should be combined with a "is_admin" if statement in the template.