Your reasoning could be applied when there is low number of users doing not-so-urgent-nor-important-things. However, the service is provided for a lot of users at the same time here, spread globally and all with their own applications and problems, and accordingly the total impact could potentially be much bigger.
Having the handicap of needing to wait 3-4 hours before being able to access your backups in an emergency, could make a day-and-night difference for continuity.
So I would argue it has nothing to do with "being a backup service", but rather that their users could afford a 3-4 hours of waiting. Or that they don't think like that.
