Based on the numbers in the article, the fake accounts generated about $1 each on average. Based on my experience being charged fees much larger than $1 by banks, it seems like the vast majority of the accounts must have generated no fees.

I can easily see how that would happen. As a bank user, some line item in my statement with $0 in it wouldn't raise any eyebrows to me. And if I did get charged a fee, they would probably remove it if I complained. Even if my complaint got escalated, what are the odds it would be attributed to a system problem and not just some idiot who forgot he signed up for an account?