Well think of it this way- they're not learning magic as much as they're perfecting thier own phsycial prowness. Adpets aren't aware of magic in the same way that mages and shamans are- hence the insult "groggy," a word for someone who's almost awakened, but not yet.
Giving adepts spell defense would make them pretty invincible- they could be as fast and strong as a troll, go toe to toe with a street sam, and shrug off spells? I'd say they couldn't have it, just for game balance.
-kv