It's not circular reasoning. The flow is clear: the Constitution gives Congress the power to make copyright law, Congress passed laws doing so, and the courts work out the details at the margins for stuff Congress didn't make explicit. Software is very much explicitly mentioned by the copyright laws that have been passed by Congress, so there's no reasonable doubt about the legal foundation there. API copyright isn't explicitly supported by statutory law, so for the courts to affirm API copyright on their own, they need to provide a convincing derivation of its existence as implicitly covered by existing statute. I doubt they can do so, and they obviously cannot do so without overturning a lot of previous precedent.
And all of the above is about the legal, technical questions about what the law is. The public policy question of what the law should be is a different matter entirely, and opinions about that generally should be addressed to Congress, not the Supreme Court.
And all of the above is about the legal, technical questions about what the law is. The public policy question of what the law should be is a different matter entirely, and opinions about that generally should be addressed to Congress, not the Supreme Court.