Article I, 8 of the U.S. Constitution gives Congress the power, among other things, to grant authors "the exclusive rights to their writings ..." for a limited period of time. (Currently, that period spans the lifetime of the author plus 70 years, but that's for another post.)
Congress has exercised that power several times. As such, the Copyright Act, codified at 17 U.S.C. 101, et seq., includes provisions that limit the liability of online service providers that unknowingly host infringed material (also known as the Digital Millennium Copyright Act), as well as provisions that restore copyrights to certain works that were first published without the required notice (also known as the Uruguay Round Agreements Act).
But in its simplest form, the Copyright Act grants the creator of original literary and graphic works (and other types of works not relevant here) exclusive rights, including the right to display the works publicly, the right to reproduce the works and the right to create derivative works from the original.
According to the U.S. Copyright Office, copyright is automatically secured when a work is created, and a work is created the first time it is "fixed in a copy." And though neither publication nor registration are required, both can still be important.
For instance, those who hold registered copyrights can sue to recover statutory damages and attorney's fees when someone infringes those rights. Those whose copyrights are not registered are only entitled to actual damages. (The difference becomes even more significant when you consider that proving actual damages isn't easy.)
But what does it take to register a website with the Copyright Office? As New York attorney Kyle-Beth Hilfer told me in an email interview:
It is complex process due to the number of potentially embedded videos, photographs, etc. in a normal website. If material is updated, it is not necessary to reregister each time. Instead, it is better to wait for a significant amount of changes and then register the website in its new format. Until that step occurs, the original copyright would afford protection of any changes as derivative works.
Just as content providers must perform a cost-benefit analysis when determining whether and how to monitor the Internet for possible infringement, they must determine whether they want to spend the time and money required to complete the registration process.