It's not just the liability. Shoplifting used to be a nonviolent property crime often, as you said, perpetrated by kids. That's changed. Many shoplifters now are hardened criminals willing to fight security personnel and sometimes are armed.
Not all stores are pushovers for shoplifters. Nebraska Furntiture Mart is the largest home furnishings retailer in the country. It was started in 1937 by Jewish immigrants from Belarus. The wife ran the furniture store while the husband ran their other business, a used clothing store. The furniture business prospered and was eventually sold to Warren Buffet although it is still managed by the family. They now have stores in Omaha, Iowa, Missouri and Texas. I don't know how big the others are but the showrooms, warehouses and parking lot for the Omaha store occupy 45 acres.
NFM is very hard line on shoplifting. I don't know whether their security people are contract or employed directly but there are credible reports of their chasing shoplifters across the parking lot, taking them to the ground and holding them for police. If they have had legal problems with indignant shoplifters, it hasn't made the news. I suspect that no lawyer would take on a rich company that won't back down in a community with little sympathy for thieves.
NFM used to post No Guns signs which have the force of law in Nebraska. They took them down several years ago.