Well, Sanc prevents effects to hand during opponent's turn, so I guess that's where the rationale is. As for when you use SoB and you have the Sanc... Well, I'm lost there.
Nothing should happen in this case. Because YOU are modifying your own hand, Sanctuary should have no effect on your drawing of cards. It's only when the opponent has Sanctuary do cards like Shard of Bravery fail.
Thematic-wise, I'm starting to think that Shard of Bravery is actually a codeword for 'Taunt'; when you use the Shard, you're basically challenging the opponent to step up their game and prove themselves battle-worthy (hence, them drawing cards) whilst you also simultaneous make a headstrong charge/tactic against them (you drawing cards equal to them). Of course, if they're immune to your provoking (Sanctuary, a clear state of mind), then your taunt fails, and there's no point in 'trying to charge' at them (can't draw cards).