This isn't so much a "all print on demand suffers from quality issues" but rather "the quality of the print on demand services is often an exercise in cost cutting."
I am certain that there are POD material that are as good as the traditional book print (and there are certainly non POD that employ similar cost cutting), but rather that in general, there is more investment in quality on the larger runs of a paperback book.
What does any of this have to do with printing? Those are formatting and typesetting issues. For the same final PDF it shouldn't matter how it's printed.
The customer could find the book they wanted on Amazon, purchase it for the same price or less via the bookstore's POD service, go have a snack and drink at the bookstore's cafe, and their book would be ready when they are done with that.
The bookstore essentially turned Amazon into a free book research services for the bookstore's customers.
> We can print millions of titles (including Google Books) that are in the public domain, as well as thousand of publisher-permission titles right to order!
The selection isn't quite the "anything that Amazon has."
They also make note of the quality with some of them that are from Google Books https://www.harvard.com/ondemand/kiosk/
> Public domain Google Books are the scanned collections of a number of libraries, including the Harvard University Library, and they may contain notations, other marks, and scanning errors. Harvard Book Store makes no representation or warranty as to the content of Google Books, and Google Books are non-returnable.