In my view it's everyone's job to help prevent crime. Yes, the cashier has the main responsibility, but if I watch someone shoplift and hold my tongue, I become an accomplice to some degree.
On the tv show What Would You Do? there was an episode where a scammer spilt water on the grocery store aisle and "slipped" on it so they could sue the store. The hidden cameras were watching the customers to see which ones would speak out so management wouldn't get falsely sued. I would want to be the person who spoke out, not the one who watched a con artist commit a crime and was afraid to get involved.