CMD.exe compatibility tab is greyed out on Win7

does anybody know why the whole tab is greyed out?

It gives the answer to that right in your screenshot

cmd.exe is a component of windows and can not have compatibility settings set.

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Even though you can not set compatibility on the exe itself if you create a shortcut to the exe you can set it to always be administrator from the shortcut's Advanced... menu.

This should give you what you want.