Free-for-all means that you can nominate any eBook as long as it is not a past winner. As long as the eBook exists in any country it's allowed.
Free-for-all does not mean the book is free or even available where you live. It just means that you have the choice to nominate any eBook. You are not limited to any specific category. It's an open ended month.
The I, Robot example is incorrect for what Free-for-all is.