You won't always have access to books from a printing press, youngster, so you must derive all knowledge from first principles!
And ftr, there's always been a good reason to learn things from first principles, and for teachers to encourage students to practice learning from first principles. You end up with a deeper understanding about it, can answer more questions on your own, and can ask better questions and get faster answers, if you understand the layers beneath your question.
That's still no excuse for teachers being dishonest about the reason though. I don't believe that teachers in the 70's and 80's thought calculators were just gonna go away.
Funny. In my day Wikipedia just came out and they used to give the same advice. In comparison, I would wager any random wiki article has a better chance of being more reliable and a better answer to your question than a Google summary.
Eh, there are entire categories of questions that can't and shouldn't be answered by searching wikipedia. A technical howto, for example, doesn't belong on wikipedia because wikipedia articles are listings of facts, not narratives about following a process. They just aren't meant for, or structured for, that type of question.
Stackoverflow also leaves a lot to be desired in that area, though, so you still need a search engine to find them.