That's not a liability issue - it's a public safety one. Stores don't have to do that as I don't believe there are laws saying it must be done (if there are some, please point them out, I'd like to read them). No one wants a kid to be snatched by some degenerate - not a store employee, not a stockholder, nobody. The security in this instance clearly executed the Code Adam poorly, which is the real problem, not that they executed a lockdown. You're not supposed to seize children that match the description, you're just supposed to "detain" them. I'm not sure if you can even really do that, as if she was determined to leave the store, they can't lawfully detain her. The whole concept is based around voluntary detainment - please cooperate with this, since it's not your problem, but we have a missing child that matches your child's rough description. They did it badly and freaked out the woman when all they had to say was "please understand, there is a missing child just like yours, so until that child is found no infants can leave the store. Have a seat by the Customer Service desk. Here is some juice for the baby."
Some stores even go so far, according to policy, as to lock the doors to everyone. Can't say I've seen that happen in person. I 100% guarantee that even though they can't detain her lawfully, had she left the store with the baby, she would have had Police searching for her within minutes if the missing baby didn't turn up immediately. Leaving the store after being asked to remain would have been just cause in that time critical a situation.