The decision on what you can do with any file should be based first on reading what conditions it has attached. Whilst legal advice is off topic, the general idea that all works are copyrighted holds internationally. As such, it is up to the copyright holder to specify what you can do with the files.
For example, in the case of Knuth's
plain.tex one can find at the start
% This is the plain TeX format that's described in The TeXbook.
% N.B.: A version number is defined at the very end of this file;
% please change that number whenever the file is modified!
% And don't modify the file unless you change its name:
% Everybody's "plain.tex" file should be the same, worldwide.
% Unlimited copying and redistribution of this file are permitted as long
% as this file is not modified. Modifications are permitted, but only if
% the resulting file is not named plain.tex.
which seems clear enough. (Note that one can argue whether Knuth's statement constitutes a license or not, but sticking to what this statement says is generally accepted as defensible.)