Justice Brett M. Kavanaugh’s first decision on the U.S. Supreme Court continues the court’s expansive approach to interpreting the Federal Arbitration Act in favor of businesses seeking to keep suits out of court.

A unanimous court Jan. 8 eliminated the “wholly groundless” exception that plaintiffs sometimes use to skirt arbitration.

The exception doesn’t appear in the text of the FAA, “and we are not at liberty to rewrite the statute passed by Congress and signed by the President,” the court...