Shoplifting is a major problem for many retailers, but it is one that they might not address by chasing after the person they believe is stealing. They are concerned about the possibility of facing legal action if they chase someone who actually wasn't shoplifting. This doesn't mean that people are free to take what they want.
It is possible that even if someone who took something is able to leave with it that they will face legal issues later. Many stores have security cameras that capture footage police can use to try to find shoplifters. Other security options are also in place at many stores. All of this can be used to further the prosecutor's case if someone is apprehended and charged.