That dialogue could be conditional to using the tadpole.
And it's not exactly resisting Shar's power, it's resisting the Nightsong's soul sucking kiss. Kiss that might also be conditional. If Shar is responsible for shielding the tadpole, then she would want you dead if you used it as well.
Makes sense, I agree. An evil goddess protecting the character from another evil entity. Overused concept, but why not.
I hope at some point though, through Gale for instance, Mystra will have a role to play to help Good leaning characters in this fight. Selune too.
