But, and I think this is really the main point, Google didn't just use the API signatures they used the API documentation to preserve the semantic meaning of those signatures which are necessary to make the API actually useful.
I think Google could argue very successfully that literally just the API signatures should fall under fair use but that's not really what this is about.
If I make a menu with the same items that your restaurant offers, it shouldn't be an issue of copyright, even if I preserve the "semantic meaning" of the menu items (copying the recipes, for example.)
Right! That's why I believe that Google's claim that it was 'just the method signatures' doesn't really hold up since they took not just the statement of the interface, but what each part does and how they interact. The only part Google didn't take was the source describing how to do it.
I think Google could argue very successfully that literally just the API signatures should fall under fair use but that's not really what this is about.