1: The teacher might be doing this out of ignorance 2: The teacher might be doing this out of insight 3: The teacher might be doing this because not everyone has high speed Internet.
If it is done out of ignorance, it is because the Internet is this big scary unknown that someone told them "anyone can edit" so they banned it. That is awful.
If out of insight, I've seen more and more students at the university-level think that an article is something they say on Buzzfeed. (Not kidding) By banning the Internet, it forces students to do more effort (sometimes literally because they have to physically travel to a library) to prove their point.
Lastly with high speed Internet, the upper-middle class were the first I knew to get high speed broadband back in high school. To this day, it isn't cheap where I live (is it for you? It is better now, but would still be a burden for many where I live) so banning the Internet puts an even constraint on students. (Before anyone mentions a computer lab, staying late at the computer lab is not the same as using wifi while you're bed or desk at home)