The reason it causes taint is that the spellbook contains secure actionbuttons. You can mouse-click spells in the spellbook directly in combat. It's basically just like an actionbar.
How secure frames work, is that its parent and any frames above it in the parent/child hierarchy also becomes secure, meaning the whole spellbook is in fact a secure frame. Changing pages by clicking the prev/next buttons initiates actions that eventually hide and show the secure spellbuttons, and that is something normal Lua can not do while engaged in combat.
Think of it this way; if it contains clickable actionbuttons or clickable unitframes, normal Lua can't show, hide, move, resize or enable/disable mouse, change hit rectangles or do anything that would affect how the user interacts with it while engaged in combat. This is to avoid addons automating or exploiting the gameplay.
Not that physically using the mouse wheel to flip through the spellbook could really be exploited in any shape or form, but that's just how the system works! Go Blizz!
