That's unfortunate. I like hathitrust a lot because it has a "go to page" function and also lists all volumes of a series on one page (whereas you have to separately search for volumes on google.books.com and archive.org). On the other hand, it seems slower than, say, archive.org for browsing. In any event, as long as the full citation is there, someone can pretty easily find the source at other websites as well.
Rick Pierpont's system of linking to a freespace page for the source that has links to all the locations for the source, addresses the problem of what link to use. But that, of course, doesn't take you to a particular page, which is an inconvenience. Some people use a link to the freespace page for the title and then a separate link to a particular site for the specific page. That kind of makes sense, but the two links are a bit confusing and you still have to pick a single site to use for the specific page.