For perspective, they found it doesn't violate the SCA, but it may still violate the Computer Fraud and Abuse Act.
Anyway, their reading of the law lends itself to a requirement to maintain (at least) two copies of a message (one the primary, and others the backups). If there is only one "copy" then it cannot be a backup.
From a utilitarian perspective, most emails I don't need to keep around once I'm done reading them, so any copy is a backup (just in case I need it again, but not because I know I'll need it again). If I know I'll need it again, then the online copy is the instrument of record.
From a multiple copy standpoint, if I don't clear my browser cache, the online form is a backup, (one could "argue").
It seems more like the law was written to be as narrow as possible in achieving its goal, as I believe laws should be. The problem is that the law's goal was very different from how it was being applied in this case, and the courts decided that instead of bending the law, they would follow it.
It was done to Sarah Palin's Yahoo! mail, and the person who did it was prosecuted. (Rightfully, IMO, despite my feelings that Palin was not VP material.) My quick review on Wikipedia confirms my memory that it was a Stored Communication Act violation there, too.