A bug report came in:

When I go to the Power Options control panel and select Edit Plan Settings, the Turn off display combo-box is disabled. Is this a bug?

Shortly thereafter, this response was submitted:

Do you see a big yellow warning over the Turn off display that says that some settings are managed by your system administrator, and a link called Why can't I change some settings?

The reply:

Yes, my machine is joined to a domain, and I see that yellow warning.

This is a fact of life. People can't see what is right in front of their face. Even though there's a warning right above the disabled item explaining that it can't be changed due to an administrative setting, even though there's a link on the page whose text directly addresses the issue they are asking about, even though they confirm the existence of the explanatory text when asked, they still don't read it or understand it. It's another case of inattentional blindness. You are so focused on the task of clicking on the combobox that you completely disregard any other information because you simply aren't expecting it to be there.