There's no rule that every homebrew class has to match with every variant.
1. A wizard who's lost his spellbook still has his spells that he memorized this morning. A fighter-type who's lost his shield has no shield.By that logic, WotC gave the wizard all-powerful spells, reasoning that as long as the spellbook could be stolen or sundered, they would still be beatable.
2. A spellbook doesn't have to be placed in a sunderable/disarmable position in order to be effective. A shield does.
3. Spells are offensively as well as defensively powerful, so the existence of a way to remove them isn't such a good balancer (plus, they can help protect the spellbook.) What I suggested is only giving the shield master high defensive bonuses, and nothing that will make it impossible to deprive him of his shield.
That's why I said it would be necessary to fix the sunder rules, so it can be disabled without being destroyed. (Stolen is not a concern, as disarming generally happens in combat, and if you win the combat you get the item back.)Players hate it when their gear is destroyed or stolen.