I don't think > it needs to be possible to use text from manuals in a program. > A manual is free if you can publish modified versions as manuals.
And is a text editor free if you can only publish modified versions as text editors -- not as manuals or tetris games or news-readers or web browsers? You have to be free to publish modified versions of the program as tetris games and news-readers and web browsers, since those are different programs, but a manual is a different kind of thing entirely. It is to much to ask that it should be feasible to conveniently publish a modified version of the program as a manual. The GPL, for instance, does not permit this in a way that is good for publication of books on paper.