I've seen this happen to people before...i.e. backpack and I think it's because of terrorism (9/11 etc.) and nothing to do with you being singled out. In a place where I worked everything had to be checked out including a bag of bagels. Backpacks seem to be targeted if people carry them into some stores.
That's unfortunate, because the location I work for has not been known to ask customers to remove backpacks or purses as they enter the store. If the store manager won't help you, ask to get in touch with the district manager.
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