I think it's just called "going out of print". At least that's what they called it in the old days. People should not effect things to remain in print forever. I think people get a little spoiled by the Internet nowadays.
Regardless of whether a company or individual releases stuff or not, they still own the rights to the material, and unless they use some form of copyright that allows duplication (Creative Commons, GPL, OGL, etc.), they have the right and prerogative to change their minds. Free does not and should not mean "Free Forever".
In this case, I can see a huge web archive like WoTC being shifted and changed over time and certain pieces of content given more or less priority. Sometimes content changes after time and things disappear. This happens even with large newspapers and even sites like Microsoft's MSDN and other archives.
El Mahdi made a good point as well. Content can take up more space than it is worth, especially depending on hosting applications.