This is a dumb ruling. It contravenes the Fifth Amendment to the Constitution which prohibits double Jeopardy and I'm not understanding why the ruling was 7-2.
Again, this is the way the Fifth Amendment has been read by the courts for 170 years. It was a 7-2 decision because the Court was literally breaking no new ground with this decision.
Yes, the double jeopardy clause bars subsequent prosecutions for the
same offense. But an offense against the United States is a separate crime from an offense against one of the states, even if both offenses are based on the same facts.
To put it another way: If the double jeopardy clause was read the way that you want it to be read, then a state prosecutor could deny the federal government jurisdiction over virtually any federal crime by prosecuting the offender first and giving them a slap on the wrist before the feds can get to them (and vice versa of course).
