In its majority opinion, the court essentially said that if lawyers brought a nationwide class action against an employer, they would have to offer strong evidence of a nationwide practice or policy that hurt the class. In the Wal-Mart case, the court wrote that the plaintiffs had not demonstrated that Wal-Mart had any nationwide policies or practices that discriminated against women.
It seems "strong evidence of a nationwide practice or policy" is sufficient enough for a big company to be found losing a battle in a class action and a massive amount of money that follows it. I fail to see the point of the plaintiffs in question having to demonstrate evidence of "any nationwide policies or practices".
Or perhaps the reason may be that in the Wal-Mart case, the plaintiffs actually based their argument on their belief that more than one Wal-Mart policy or practice harmed its former and current employees, and here the writer merely means to report the facts.