You get automatic copyright on stuff you create, but registering gives it a little extra security. I know when I submitted my books, they put a copy in the archives and give you a pretty form (TX-40 is either the one you get or the one you submit) that proves that you submitted. Publishers requests you do that before they'll print your book (at least from my experience).
This is a little different. It applies to all published copyrighted works, not just works that are submitted for registered copyright (though you can submit a combined registration request and Library deposit).
And copyright verses non-commerical are two completely different things. You can still have copyright on something and give it away. At least, from my own understanding.
Entirely true. I thought there might be a non-commercial exemption because it does sound like the law is designed for (commercial) publishing houses, and I'm not sure how the Library would deal with the number of submissions it's supposed to be (?) receiving.
Even if there were a non-commercial exemption, there's still a ton of new copyrighted work being self-published right now.