Since "in the Internet" is the literal translation of "na Internet", I can understand that it doesn't seem unusual to you. Perhaps the Google results for "in the Internet" are from non-native speakers who translated it literally from their own language?
Finding an authoritative explanation isn't easy. This is the best one that I have found (from a couple of journalists and editors):http://www.grammarphobia.com/blog/2012/11/in-vs-on.html
"By the late 800s, though, people began using 'in' more loosely to refer to being within things like a book or the writings of an author. The preposition is being used in this sense when we say something is 'in' a movie or an article or a newspaper.
In the late 19th century, English speakers started using 'on' to refer to a medium of communications—the telephone at first, then radio, television, and the Internet."
And here's my favorite non-authoritative answer:
