There are good reasons why we restrict powers like arrest to trained agents of the government. While there are certainly many problems with the conduct of police and oversight over them, at least they have training and experience and there are mechanisms in place to evaluate their actions. By contrast, empowering every shopkeeper and random citizen to physically detain people who they think are criminals seems dangerous and unnecessary. In a few cases, it may be the least bad option available, but I think the onus should be on the person performing the arrest to justify it later.
In the grand scheme of things, shoplifting seems a lot less significant than physically detaining somebody against their will. Saying that as soon as somebody steals from you, you have the right to effectively kidnap them seems liable to create harm and abuse. Kidnapping is rightly considered a more serious offense than shoplifting, and I don’t think the fact that someone committed a crime before being thus apprehended has all that much legal or moral significance. It smacks of the sort of crude revenge-based legal systems where people get their hands lopped off (or get thrown into the terrible conditions of prison, but that is another discussion).
That’s why I think it is wrongheaded when people argue that David Chen – the Toronto shopkeeper who physically detained a shoplifter – should never have been criminally charged. When you opt to take the law into your own hands, you are effectively claiming that the situation is so important and so urgent that you should take over from the actual authorities. It seems to me that such cases are rare and involve things like real risks of injury or death – not the danger of losing a few dollars worth of merchandise.
If you feel that you need to usurp the powers of the police, it just seems sensible to expect that you may need to justify that choice in a court of law. They may well find that you behaved reasonably. But the fact that there will be some after-the-fact oversight could in itself act as a minor deterrent to abuses of power.