Interesting. I've never heard of that.
So I stand corrected. (Sort of.) Apparently it's a very old provision. [1] It doesn't really make much sense in today's world. Read literally, almost anything you create--including online--must be sent to the LoC.
So, yes, it is a law on the books and maybe mainstream publishers comply with it. But But pretty much no one else does AFAIK.
[1] http://articles.ibpa-online.org/article/need-know-copyright-...