I get where you’re coming from, but I think the point is that if you gave a test and the students could pass it using Google translate, it wasn’t a very good test. Same thing with the book. If the students could pass it by looking at the book, what’s the point of the test?

Now for a language teacher there would obviously be some exceptions here. If you did a quiz that was strictly on vocabulary, maybe we need to shut the books. But that’s because at that moment you’re specifically testing students on whether they have internalized the necessary vocabulary. Most of the time when we are testing, we are testing for understanding, not rote memorization