Because the document you sign says that you cannot share this product with non-disclosed individuals until after first customer shipment.Okay, again, Apple announced it after the keynote. Videos were posted online of it working. So why is this so different?!
You'll note that every individual that was authorized to share a video about the phone was at Apple Park, and every unit was exactly the same. Very likely PVT (Production Validation Test) units, or even the first few Mass Production units. However every unit is "press ready" - no secret info, no chance of odd production variances, fit and finish issues, or software that may be perceived to misrepresent the product or it's promised performance.
Every one of those devices was likely individually signed off before being allowed to be demoed. This isn't some sort of thing where Apple just unboxed 100 iPhone Xs and set them up in a room. It's staged so that stuff doesn't happen. The demos are setup to show the product in the best way possible - it's a marketing event, not a "do whatever you want" day.
If they didn't? Well, we all remember what happened when Face ID didn't work when Craig went to demo it. Instant speculation on how terrible the tech is, and how it's not ready for prime time.
Chances are he's holding an EVT (Engineering Validation Test) or DVT (Design Validation Test) unit. Imagine she posted a video where dad's new phone didn't unlock correctly for Apple Pay because of whatever bug they were working out with that unit. How would that represent the company, in an uncontrolled environment? What would have happened if there was some lag due to a developer build on that device, that all of us hungry MacRumors readers would then spend the next week analyzing how buggy the device was. Not something Apple wants to leave to chance.
He was lucky nothing like that happened - that's when damages would be rendered. Apple would have easily then said he irreparably damaged the product launch and brand, and he'd be facing real financial consequences. Instead, he was simply in breach of the agreement - when you get that device to carry, you sign your life away and are told to protect that product. He disregarded that, and when they found him, they terminated him. He knew he had no leg to stand on.