Because she seems to be the source of this magic, not Shar.
Note the end of her official description: "...while threatened by a strange new magic that is burgeoning from within."
She is probably some sort of sorceress and does not know it.
Right, she's the source of the light. And guess who's the patron goddess of (moon)light? Selune. Shar's sister and according to Forgotten Realms' lore, the other half of the primordial deity that created Abeir-Toril (the planet where the Forgotten Realms are located). Considering how the school journal points out at students disappearing (in a village dedicated to Selune, no less), how Shadowheart mentions that those events are the work of other Sharrans and how Shadowheart herself confirms she was brainwashed in order to better fulfill her mission it seems to me her being a Selunite that was captured and brainwashed by Sharrans to be a more likely and meaningful explanation of her circumstances rather than her just being a light sorceress without knowing it.
