Why the hell not? It's a proper noun, and we capitalize them. The proper noun "Internet" refers to a specific instance of an "internet", a common noun.
But in real-world usage the word "internet" always refers to the internet. You would never refer to "an internet"—more likely terminology would be "intranet," "LAN" or "corporate network," or, in a historical context, something along the lines of "internet precursor."
Going further, "internet" is wrong. There have been, and still are in some cases multiple internets. The Internet may dwarf the others in size, but the distinction is still an important technical point (AP rules be damned).
Technical correctness is moot when the majority already (mis)understands a word's meaning. Regardless, "internet" is just a laughably generic abbreviation of "interconnected (computer) network". On the technical front, I would think WAN/MAN/LAN would be the more proper technical terminology, right?
This whole conversation reminds me of the unwinnable fight to get popular media/culture to use "cracker", rather than the over-used and technically incorrect "hacker", when malicious black-hattery is afoot.
Technical correctness and precision language is what enables those of us who build the future to progress forward. Incorrect language and imprecise thinking holds back those who do not clear their minds, preventing them from contributing themselves to the global project of progress. These things do matter.
Sensible defaults are not a harm to technical correctness or precision. It's not like anyone would be able to refer to any of the other internets without at least a qualifying adjective, and it would likely require much more context than that.
1 - I'm torn on how to handle the evolution of language. On the one hand, language evolves and mutates. To complain that people use "literally" wrong is to shout at the tide. On the other hand, words have specific meanings, and if we discard those for different meanings, what takes up the slack? If "literally" now means "emphasized figuratively", what then would I say to use the old meaning? If "meme" now means captioned pic, how do I refer to a concept transferred between people? I really don't mind language evolving, but the loss of precision bothers me.
2 - Re: the "unwinnable" fight to promote "cracker" over "hacker". I'd argue that this fight had some success! Not that anyone says "cracker", but that the definition of "hacker" moved from "treacherous programmer" to "skilled computer programmer, which allows for the possibility of threat". No idea what the dictionary says, but when I hear "hacker" used in the media these days, it _feels_ different than it did in the late-90s, early-aughts. Perhaps that change would've happened without the attempt to emphasis a difference, but I'm inclined to believe the effort had some payoff.
I fear that today, most people's understanding of the Internet is the Web or, worse, Facebook. This isn't necessarily a misconception we want to promote either.
Which "things" constitute proper nouns is at least a bit ambiguous. In general, there's a tendency to capitalize new terms that haven't entered the broad lexicon. So, at various times, it's been common to see the Web, Big Data, Open Source, and so forth. The problem with doing things this way is that you either 1.) End up dropping the caps semi-arbitrarily at some point or 2.) you end up with sentences like "I commonly use Open Source Software on the Web to do my Big Data analyses."
My usage of the internet really isn't that much different from my usage of the automobile. I don't know why you think it's a proper noun. It's just the name of the thing.
