Thats what patents are for. API is like designing your own custom plug to your device. You can't copyright that plug design, you can patent it its novel and new.
And if you are opposed to algorithmic/software patents it would be an unpleasant choice to make. Also, copyright is probably (I'm not sure) easier/cheaper to litigate, because it's much more evident when something is copied, compared to divinating whether something violates a patent or not.
