OTOH there are four critera for a legal stop -- they need to see you enter without the merchandise, select it from the shelf, conceal it, then walk past the point of sale AND all merchandise. And you have to have an unobstructed view of the person, because if they discard the item you stop them for, you're in for a world of (legal) hurt.

Also many stores have shot themselves in the foot by placing items for sale outside the front doors... thus a shoplifter could claim they just stuck something in their pocket because they forgot they needed a pumpkin and thus needed a cart, or something to that effect.

If you stop someone and can't document these four points, they can challenge the stop, and you're up for a LOT more losses from the unlawful detainment suit.

So basically, they value upselling people at entrances more than limiting liability, and a savvy shoplifter can sue for a lot of money if the store allows reusable bags, since that removes the ability to charge for "concealment" given that by selling Safeway or whatever branded opaque bags, you have implicitly consented to "concealment" of merchandise.