Does anyone know why, even though when you post a book without an ISBN date, there is a place to fill in the publication date, when you bring up a book posted without an ISBN, the publication date doesn't show? I find this really annoying as I like older books and sometimes I specifically want an older edition as opposed to a newer one.

I'm not really sure. Does it never show? It could be that some folks just don't take the time to fill that data in ...? (Too bad if that's the case.) But I really don't know if PBS will accept the no-ISBN posting form if the date is not filled in, so can't say for sure.

I just checked my own shelf and the books that I entered a publishing date on do show it, but there's at least one that doesn't because I couldn't find a reliable enough date in the book.  Sounds like you're just finding books where the poster didn't bother entering it.