Fair or not, GCG gets to tell you what you can and can't do to your books. In fact, they're not even your books. You are just holding them for GCG until you slab them, at which time (if you read the fine print), ownership in the book switches to the GCG. That's why you can't spray your books, I mean GCG's books. GCG doesn't want you spraying their !
You are adding a physical composition to the book that was not part of the manufacture. Pretty simple, realy.
What about the oils from my skin when I touch it? Or humidity in storage? Pen marks and glue are considered resto depending on the assumed intent. Tape is sometimes considered a defect, etc.
It's completely arbitrary.