There are applications which automatically save images sent to you to your camera roll (such as Whatsapp, IIRC). How can Apple prove you put them there intentionally?

Granted, they most likely won't care, but it's a legitimate attack vector.

You’re right that it’s a valid attack upon the people Apple pays to review matched images before invoking law enforcement, but no harm comes to the recipient in that model, unless they receive real legitimate CSAM and don’t report it to the authorities themselves.

Attempted entrapment and abuse of computing systems, which is an uncomfortable way to phrase the WhatsApp scenario, would be quite sufficient cause for a discovery warrant to have WhatsApp reveal the sender’s identity to Apple. Doesn’t mean they’d be found guilty, but WhatsApp will fold a lot sooner than Apple, especially if the warrant is sealed by the court to prevent the sender from deleting any CSAM in their possession.

A hacker would say that’s all contrived nonsense and anyways it’s just SWATting, that’s no big deal. A judge would say that’s a reasonable balance of protecting the sender from being dragged through the mud in the press before being indicted and permitting the abused party (Apple) to pursue a conviction and damages.

I am not your lawyer, this is not legal advice, etc.