I'm a stickler for words. Perhaps that's where people seem to think my "hangup" lies. Prevention and deterrence are 2 different things to me. The word prevent means to keep something from happening whereas deterrence means "the use of punishment as a threat to deter people from offending. Deterrence is often contrasted with retributivism, which holds that punishment is a necessary consequence of a crime and should be calculated based on the gravity of the wrong done." That box hasn't convinced them not to steal, it may have convinced them not to steal from Tesco however. That's more of a matter of ease though. CBT is preventative because you attack the source of the problem - the thoughts that lead to risky behaviors. Kind of like killing a weed. You can spray what's above ground all day long and call it preventative but until you get to the root of it, it will always come back.
The size of a product contributes to the ease of which it can be concealed sure, but that doesn't mean that it limits theft to small objects or makes it less easy to steal something larger. I can say with confidence that if someone says to themselves, hey I want to steal an iPhone 6, that a couple of plastic straps and and alarm will not stop them. "Thieves who are coming in to stores geared up to steal are looking to make the most amount of money with the least amount of risk." Yes, but that doesn't mean the item can't be stolen or that it can't be walked right out the front door. I know people have impulse control issues and steal in the heat of the moment but again, this box doesn't prevent theft. I could say it might deter someone to steal something else instead. So the point of it, for Tesco not to lose money through shrinkage, becomes moot. Lego has already been paid for the merchandise so they don't lose out on anything but Tesco still can.