Obviously the store's policy is designed to prevent theft, not terrorism. They believe (maybe wrongly) that a lot of their "shrinkage" is due to young students who hide stolen articles in their backpacks. They can't fairly enforce a rule against minors while not enforcing it with adults. That's why they don't allow backpacks, but do allow purses. I know, I rail against it too, and complain when they ask me to surrender my backpack. Most employees let my 50 year old ass in the store with a backpack, but there are a few gung-ho martinets who insist on strict enforcement.

To make it less likely that they will ask for my pack, I put it in the cart as soon as I walk in. If it isn't on your back, it isn't really a backpack, is it? I've thought about acting really gay, batting my eyelids and saying, "Oh no sweetcheeks, I simply must hold onto my purse."

I would not leave any valuable in the pack--valuables meaning cash, credit cards, anything that a store employee might covet. But I don't worry about them taking my other stuff, like clothing, tools, inner tue, etc. Usually security is watching the employees more closely than the shoppers.