It seems they were trying to purge unused accounts from their system, which is perfectly reasonable. Whatever their checks and balances were to prevent legitimate accounts from being wasted along with it obviously didn't work.
To make up for their mistake, they are offering a $50 credit to affected customers. That's probably not going to make those people who relied on those email servers feel any better, but it is another great argument for backing up important data. As one of the largest ISPs in the U.S., you would think they could afford the technical expertise and data backup solutions to make sure things like this simply can't happen. Regardless, they claim it will never happen again.