The feds always base their jurisdiction in these cases on mail fraud (something in the case was mailed) or on wire fraud (use of interstate phone calls, faxes, emails or bank wire transfers).
It's virtually impossible to do any business transaction-- legitimate or otherwise-- without using the mails or "interstate wires."
I understand. But Scruggs and the Judge are in the same town, Oxford. And it seems to me that Scruggs et al. surely would have been smart enough to know that they should approach the Judge only in person -- never over the phone or via post.