[tex-k] Is TeX in the public domain?

[胡亚捷 (Hu Yajie)]

The copyright page of Volume B states that "The program for TeX is in the
public domain," and http://tug.org/texlive/copying.html also says so.
Well, we all know that "public domain" means "no copyright restrictions",
yet tex.web has a copyright notice that forbids copying modified versions,
not even if the file is given a different name. Are they contradictory?

The more lenient "Knuth License" on ctan.org/license/knuth does allow
modification as long as the name is changed. This seems to be another
contradiction.