Sounds logical, so what's the problem? The problem is that it gives employers the incentive to cut off a class action at the knees by simply paying the plaintiff her damages in order to moot the entire case. Another problem surfaces when, as in this instance, the plaintiff rejects the settlement offer. If the offer is rejected, isn't there still a case and controversy?
Justice Kagan seems to believe there is still a case. In her dissenting opinion, she blasts the majority. I highly encourage you to read her dissent, as well as an analysis of it here and a general analysis of the case here. I abhor this decision. What say you?