Apple - App Store: "To update this app, you need to accept it on the Account page"

While Simon's answer is correct, the reason this may be appearing for you is actually because you have those apps pre-installed. I ran into a similar situation just now. Despite already having those apps tied to my Apple ID, there's something about this having them pre-installed on my machine that breaks down.

Simply delete those apps. You'll be able to redownload and update them from that same "View My Account" page with no issues after that.

Tip: For those unable to reinstall (work computer, etc.) if you simply delete then "Move Back" from the trash you will achieve the same result.

I found it under Store > View My Account.

Then click Accept.

Apparently this occurs when you have a new Mac that has certain apps (like Keynote) pre-installed, but you have also purchased them at some point in the past. So you are "adopting" the pre-installed ones into your Apple account.