As do I. For those, and the majority of my GA collection, I use archival "bags and boards" (like Mylites). They are as good if not better for archival purposes, make the colours really stand out, take up less space and (best of all) give me a chance to take the book out to look inside if ever I want.
The book in the link was slabbed recently, it didn't stick around long in anyone's collection as a slabbed book. So my money is on someone making a mistake and not looking inside the book before sending it in.