To make it a matter of public record as at chambers decisions were not published as a matter of course back then. It was clearly marked as an at chambers decision of the Chief Justice, not a District Court decision.
So you're saying Supreme Court decisions aren't matters of public record? I have a hard time believing that. Are the the workings of some Star Chamber? Made in secret and kept secret? Something the public has no right to know about? Hardly. Taney filed his decision with the Maryland District Court because that was the court where the matter lay, not becasue it was a Supreme Court decision that somehow couldn't be published.
And what about the jurisdictional question? You insist that Ex Parte Merryman was a Supreme Court decision, yet according to Article III the Supreme Court had appellate jurisdiction in this matter. So who rendered the original decision? And a Supreme Court decision handed down by a single justice speaking for the entire court. As I mentioned earlier, and at the risk of admitting my ignorance in this area, I'm not aware of a single Supreme Court case decided by a single justice. I always thought Supreme Court decisions needed a majority of the justice to agree, yet you say that single justices can act on a case alone. Can you provide me with other cases where a single justice issued a decision for the court so I can improve my understanding of the judiciary? I would really appreciate it.