The impression I got from the episode, but certainly not the only reading of it, is that it's about verisimilitude. If you just delete the book, then you've changed the simulated world artificially, potentially rendering it's academic value as a model moot. The whole point is to learn about humans, Earth, and how to best conquer the planet. How much good data do you get when you just cheat the game world to get the result you want?
Thus they have to physically intervene in the situation, as they would if it were a real event, in order to keep the model untainted.