I wondered this a few months ago, my "restore from backup" was greyed out too but never found an answer.
However I have since discoved that after a factory reset or even when my phone was exchanged under warranty that the HTC backup will rebuild your One exactly how it was when the last backup was made, by this I mean the settings, passwords, accounts, apps, even your homescreen will be built including folders and wijets, theapp draw is organised too, such as any folders you may have created there.
I have used it twice now at it does a great job, once you turn you phone on and sign into the HTC account that you backed up with it is pretty much automatic ( it prompts you to log into the dropbox folder when required). What you must not do is use google restore if prompted and it will not restore app data such as level progress or achievemnts in games or passwords etc. I think the backup creates a list of apps that are then down loaded from the appstore in the background rather than backinging up the app itself.
To recap my restore from backup has always been greyed out but when needed after factory reset or setting up a new device it has worked fantastically.
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