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.