Hello,
An administrator of the server that has SharePoint installed will automatically be granted access to all the sharepoint sites without having to even be added as a user in SharePoint, this is done by default and it does not just apply to SharePoint.
An Admin (Domain Admin) will have full rights to everything on a server (for example if you have a file server, the Admin will always have full rights to everything, unless that is configured/restricted manually on explicit folders).
In organizations I have been working and and been an Administrator we have had to sign non-disclosure agreement, stating that admins are not allowed to use information they have access to that might harm the corporation in any way, etc. etc.
Regards,
Thomas