Deleted Characters Being Reviewed

Surely they should have done a thorough database back up before running a query to delete multiple characters...

It isn't the fact that they ran the query that bothers me, it's the fact that they can't reverse it.

They can. It's what is colloquially known as a "rollback", and is actually a restore from savepoint/backup.

However, they did not decide to do this as soon as the problem came to their attention, about two hours after server-up. Why?
Nobody knows. I'm going to go with "they went to ask permission from some head exec, who roared 'we don't rollback!'" for various reasons of corporate pride or w/e.
As time went by, with the game open, more and more things were changing. People were leveling, scrolling, cubing, trading, buying stuff in cash shop and mts, charging NX (these last are particularly troublesome because cash-related databases are separate from game database and linked to the real world), and so on.
The longer the game stays up, the more problematic it is to "roll back".
Eventually they decided to try and copy only the data of the deleted characters from the backup, instead of overwriting the whole database.
This is what they are doing now. Gotta pray real hard they don't foul it up, because they have never done it before.