Okay, if this article isn't evidence that the "new" security screening protocol in airports just plum doesn't work, I don't know what is.
Had I been that woman (though I couldn't be, as I don't own a stun gun and doubt I would have been brainless enough to leave it in my purse before boarding a plane in this day and age if I did) I would have taken the same position I do on those rare occasions when I get home from a store and find that the cashier didn't ring up one of my items, but put it in the bag anyway: I did what I was supposed to and in no way did anything purposely to act in a criminal way. There's harm because the store loses revenue, but it happened because of an error by their employee so I don't feel compelled to go all the way back there and say, "Hey, you didn't take my money for this!"
The same way I didn't intend to deprive the store of a sale, the woman knew she didn't intend to hurt anyone using her weapons. Calling it to the attention of a flight attendant was like holding up a sign that says, "Please arrest me and make me fill out all sorts of paperwork and spend a bunch of time being questioned!" Okay, she wasn't arrested, but I can't imagine that being taken "into custody" feels a whole lot different when you're an average citizen whose knowledge of the criminal justice system is comprised of watching all three flavors of Law and Order every week!