As the quote in your question notes (albeit in contrapositive form), Asimov's fiction may be made legally available for free download on the net with the permission of his estate.
It is either that case that
- Princeton University's physics department (or whatever individual person or agency put up the story online) has obtained permission from Asimov's estate to distribute the story on their website, or
- they simply distribute it without legal permission, and Asimov's estate has not taken legal action to have it removed, which they may or may not choose to do in the future.
Consider also that the Princeton distribution does not appear to have been meant to be in any way public: looking one directory level up (Princeton University Physics 115A and B: Physics for Future Leaders) shows a course overview with syllabus and lecture notes, without any apparent link to the story. Probably, it was put online for use exclusively by students taking that particular class, and the professor(s) just didn't put any controls to stop it from being accessible to anyone who knew the URL. This doesn't really change that fact that its public accessibility is, strictly speaking, copyright infringement, but it may be a favorable factor toward a finding that distribution outside of the class was innocent infringement, which can lessen (or, rarely, nullify) the penalty. (And it may be quite possible that distribution within the class was defensible under fair use/dealing, which considers educational context as a favorable factor.)