Yes, it's all about ME, me the user, me the person at the other end of the connection. I'm also not just any user, I am an advocate of libraries, a librarian, and I made the effort to go to HathiTrust -- a site that has not shown up for me in search engines.
This seems to be such a basic lesson that I do not understand why libraries can't learn it. User-friendliness.
Ooof! It just gets worse. I decided to see what login is about. To get to login, you have to search, select a book, and click on login. On the book page, you may see that a book is "Public Domain" or it may say "Public Domain, Google-digitized". When you log in, you log in either as someone from a member institution or a guest. The guest log in form states:
Does NOT provide access to full PDF downloads of public domain & open access items where not publicly availableHowever, it turns out that it DOES provide access to PD books (see comment by anonymous) if the book is not digitized by Google -- but that isn't what you've been told. "... not publicly available" isn't what you see on the book page, you see "Google-digitized." The page on policies has two different categories, "Open Access" and "Open Access, Google-digitized." Nothing in the definitions of those categories mentions member and guest downloading.
Basically, HathiTrust turns out to be a tiered system with member and non-member access. You don't encounter this until you try to download something that is PD but not "publicly available." Nothing on the home page mentions that this is a member-based service, therefore you don't know that as a non-member you will encounter walls.
OK, it is resolved, that from now on I will always go first to the Open Library, a site where Open means what I think it should.