Just had a daily reward give me a Fog Buster when I already had 100% of map revealed, and because I already had 5 in my inventory, it forced me to use it, and of course it did nothing (b/c there was nothing left to reveal).
That shouldn't be possible. Are you sure there wasn't one territory hiding somewhere that was still fogged?
If you try to use a fog buster when every territory is revealed, you'll get a message saying you can't use it.
Daily bonus will never give you a power you can't use. It won't give you a market raid if you have no markets. It won't give you an Inspire Merceniares if you have no mercenary camps. It won't give you a supercharge mine if you have no mines. etc.