I don't drive and always carry a backpack, so I've encountered this many, many times. I know it's done for security, and all that, but after a few years it gets really old.
Lately I have noticed many women carrying handbags as big, or bigger, than my backpack. Usually these oversized handbags are left open and placed in on the shopping cart where a little kid would sit. My backpack is zipped up and being worn with both straps. In my opinion it would be much easier for a woman to sneakily place an item in her bag than it would be for me to take off my backpack, unzip it, and put it back on. How this is not discrimination I don't know, but I guess being a white male does not allow me to make such a claim.
Sometimes I may carry something of value in my backpack. Why should I trust the minimum wage employee with a tattoo on his neck with my digital camera?
Also, there is a grocery store close to my house that sells these big canvas bags to the "earth friendly" people that don't want their groceries in a plastic bag. Although I've never seen anyone use these bags, I'll bet they don't have to be checked upon entry into the store.
Sorry for the rant, but like I said, I've been thru this many, many times and it gets old.