I think store security people have the peception (true or false, I don't know) that young students do a lot of shoplifting. They ask all customers to check their backpacks so that they can ask the students.
I usually point out that it doesn't make sense to take my backpack, then let (usually) female customers shop with their big handbags. The greeter usually agrees--they would really have to agree, since it makes complete sense. Then they sometimes lt me proceed with my backpack, sometimes they don't. I dont' really think the employees are going to steal anything, I mean you would have to be a tad paranoid to think that, wouldn't you? So, I'll give it up if they continue to insist, but I do make a minor stink about it. Like james, I don't try to make the employee feel bad, especially since they have always been courteous to me.
Something to think about: Do you believe that those security cameras are all real, or that they all work? Some are hoaxes, and many are not hooked to recorders.
Now, the whole issue is a much bigger problem at high security locations like courthouses and federal buildings, where you have to pass through a metal detector. My tools have held me up pretty badly there. Sometimes they tell me to go outside and hide them, especially an all purpose tool that folds like a pocket knife. I have better luck since I started keepin the tools in a little pouch in my backpack, instead of scattered all over.