Well noticed Tradutora, it can take an "IN'' preposition when the "Internet" is an adjective, that is, when it is specifying a noun or something else.
The examples given show exactly that, as in: in the Internet age, in the Internet jungle, in the Internet underworld.
Worth noticing, that you covered at least two topics on that one. Preposition usage and capitalization (since when "internet" defines something else, it takes
a capital "I".)
Thank you guys, all of you, for the teamwork.