News like "zero-click iMessage to root exploit" costs Apple millions of dollars as it's a PR nightmare and erodes the trust they spent years (+tens of millions of dollars) building. Those "thousands of dollars" they would spend on forensics would be their least concern.

Imagine The Guardian, The New York Times and several other top journals covered this story with a sensational title like "Our Journalists' iPhones are hacked remotely". There's no going back from that.