Because they A) could refund us for discontinuing the service we paid for or B) transfer us to the new service. The new service includes 20GB, thus by transfering the service to mobileme, we get the 20GB. Thus they are not giving us anything.
Giving us something would be upgrading everyone by another 10GB because of the outage, or refunding us some money (which would be pointless as it would only add up to pennies or a few dollars).
You had 10gb, you are automatically put on a new service with 20gb, so you now have 10gb more... thus they've given you 10gb more than you had before... why can't you see that?