Suppose the question is why even use the tool if you can tell by looking.

I'm guessing it's because if you just say it, they'll argue or maybe get their parents to argue and if you can have a 'detector' serve as 'evidence'? Especially if they believe in the tool to use in the first place, they are likely to believe in the detector tool, no mater how flawed? Or to just serve as a general deterrent, your work is being checked by a machine and you'll get caught if you try...

I'll say at least some kids are scared of being falsely accused of cheating because they don't trust the tools to be accurate, perhaps more so in the very anti-AI kids that would avoid the AI like the plague. No perfect answer how you reassure that the tool isn't immediately trusted without also defanging it to the intended audience in that case though...