The problem is that this comes down to who you believe. Any thief would use the exact explanation that this kid used. "Oh, gee, it didn't check out, but look at my screen– I meant to buy it." The mentality is, of course, create a plausible excuse and best case scenario, you get a free pair of $250 headphones. Worst case, you're caught and have an excuse– having to buy them like a sucker (everyone else.)
The obvious problem is that this same scenario could be a legitimate error– the kid could have actually made this mistake innocently. But here's where profiling comes in to play. If this happened to a middle aged consumer that looked like they had the means to otherwise buy this product, security might buy the story of a mistake. But if you are a scrappy looking kid from Jersey, who doesn't look like they could afford it, and this happens– too bad!
Ain't saying this is the wrong approach, but this type of thing happens in retail all the time.