They could easily get this done in India or China. If they are really doing book scanning, I'm shocked this isn't being done in the far east. With that perspective, this isn't far from "don't be evil".
It's not the best practice in the world but this isn't exploitation.
Perhaps it should inspire some investigative journalism. Perhaps Google could fund a program and give these data entry people the opportunity to innovate, rewarding them accordingly with a small scale founders' award?
Despite the assumed background of these employees, they're at Google inspiring their kids. With so few benefits outside of their salary, they can always work elsewhere. But why would they take fast food or something of the sort over this?
I imagine the cleaning crew that vacuum at Goldman Sachs at night get treated way worse.