United States v. Trans-Missouri Freight Association

United States v. Trans-Missouri Freight Association, 166 U.S. 290 (1897), excerpted in Trade Regulation text, at 51. On 53:

"We are asked to regard the title of this act as indicative of its purpose to include only those contracts which were unlawful at common law, but which require the sanction of a Federal statute in order to be dealt with in a Federal court."

The Court (Justice Peckham) rejects this argument, but not on any kind of structural argument, but rather on a statutory interpretation principle. Regardless, though, even 40 years before Erie, it appears that it was not unheard of to argue that a federal court couldn't just go around making law outside the realm of federal statutes. Why this argument would have been viable isn't clear to me -- didn't federal courts make law outside of federal statutes all the time? Isn't that exactly what Erie put a stop to?