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").