I think a few things at play. OneDrive isn't really a backup alternative (although there is an opportunity fro somebody to write a backup program that uses onedrive as the destination)
I bet the vast majority of customers only use a fraction of the 25GB so this actually probably doesn't have a big impact on their usage. Especially since it doesn't really have the sync like features of Sugarsync.
This is a race to the bottom to getting customers to sell additional stuff. Can't see how this is very profitable although the cost of 1TB of storage to Microsoft hardware wise is probably about $40-$50.