Presumably they found it more important to preserve the pages than to preserve the volume binding. The FAQ for the linear book scanner indicates that Prototype

Presumably they found it more important to preserve the pages than to preserve the volume binding. The FAQ for the linear book scanner indicates that Prototype 1 mangles pages in some way (tears or folds) in about 45% of books it scans. Moreover, it used to be common to create bound volumes by binding multiple issues of a serial together. (In some cases the bindery would crop pages to fit!) The binding is often so tight that the volume cannot open flat enough for a full scan. Separating the pages from the spine allows for the entire page to be imaged without distortion. Edit: Incorporated correction. I had accidentally stated the claim more strongly than the FAQ supported; however, my point does not depend on the claim being as strong as the form in which I had stated it.