My family runs a retail store in Ontario, Canada. The laws here are probably different from where you are, but here are the main points.
Shoplifting is just a form of theft, although usually involving items of lower value. The Criminal Code differentiates between thefts of goods over or under a threshold value, the main differences being in severity of treatment upon conviction.
A store owner or employee has NO right to physically detain or assault an alleged thief. The fact of theft alone is not legal justification for such action. Now, if the thief assaults store staff in the incident the victim certainly has a right to self-defense as long as it is a "reasonable" level of response; still, theft alone is not cause for assault in retaliation. Technically, store staff who physically grab or detain a suspected thief, or search through their clothing to find a stolen item, are themselves committing assault. In practical terms, VERY few thieves will try to pursue assault charges against store staff in such cases, and police officers summoned to the scene are likely to downplay the "assault" aspect so it never gets processed. The same holds true for Security staff in the store or mall. These people are NOT deputized police officers with a right to physically detain suspects. However, they do it all the time and depend on the fact that police usually will back them up and ignore their actions as long as the suspect was not seriously harmed.
Making a charge of theft stick in court can be tough. Basically the store employee must be able to testify that he / she saw the accused pick up the item, conceal it (or not) and maintain possession of the item until the accused has left the premises without paying AND until the accused has been stopped and found still to be in possession of the stolen item. If you see the item taken but the thief manages to dump it before being caught, there is no proof of the theft. If you don't see the initial theft and only suspect it, finding the item in the thief's possession after leaving is proof only of possession of stolen property, and not of the original theft itself.
In the aftermath of such incidents many merchants are greatly disappointed by the light punishment given to offenders upon conviction. The one thing they can do to protect themselves against repeat performances is to issue a Trespass Order. Any private property owner (and this includes the tenants of leased retail space) can issue an order to any person (with reasonable cause such as having observed the person attempt to steal) not to enter that property for some specified time, even up to never again. Once that Trespass Order has been given to the person, he / she can be arrested for violating it on any future attempt to re-enter the store.
