APIs have not been ruled uncopyrightable. The Supreme Court found that Google's use of the Java API's was fair use; fair use is specifically a defense against infringement of a copyrighted work.

There may be court cases in the future that determine what the boundaries on API reimplementation are that distinguish fair use from infringement. A future Supreme Court may well overturn Oracle v. Google. APIs are specific forms of unique expression, and the same functionality can be made available through different APIs. (See for example, OpenGL vs. Direct3D.) Typically these are the criteria used to determine what is eligible for copyright, and ruling APIs uncopyrightable absent a statutory carve-out exemption may well put the copyrightability of currently protected forms of expression in jeopardy.

But as things stand, the Oracle v. Google decision has only made the API-copyrightability decision more ambiguous, it has not settled the matter in favor of making APIs uncopyrightable.