Let's take a case in which a plaintiff files a putative (uncertified) class action that's removed to federal court under the Class Action Fairness Act -- i.e., because there's minimal diversity and $5 million-plus at stake. The district court thereafter refuses to certify the class. Does jurisdiction still exist in federal court, or does the lawsuit get remanded?
There's a very simple foundational principle that answers that question: As long as jurisdiction exists at the outset, it persists. You can muck that central principle up with a variety of complex arguments, but that's basically the rule.
So the Ninth Circuit gets this one right, and properly recognizes that the district court got it wrong. Even after a refusal to certify, the case gets to remain in federal court.
There are additional policy and other reasons why this result should follow as well, but you don't even need to get to 'em. Sometimes the most straightforward part of a cases provides a pretty good answer itself.