Fear. They make a big deal about punishing leakers to make people fear what will happen if they leak.
Also control. Here is a story I heard from an iPad launch partner, who got an iPad before it was announced:
The device arrived in a large box with little plungers sticking out to press the buttons and the screen. You couldn't actually see the device inside, so there was no way to know what buttons were on it, what its dimensions or weight were, etc. Furthermore the box had to be kept in a locked room with strict access controls. Anyone who had access to the room had to be under draconian NDA that included severe personal penalties for leaking anything (like enough to financially ruin you). Also everything was watermarked so you couldn't release photos without them knowing exactly where it came from.
The box was taken back and replaced with a standard consumer model after the launch. Most people in the company weren't even aware they were a launch partner until after the announcement.
Also control. Here is a story I heard from an iPad launch partner, who got an iPad before it was announced:
The device arrived in a large box with little plungers sticking out to press the buttons and the screen. You couldn't actually see the device inside, so there was no way to know what buttons were on it, what its dimensions or weight were, etc. Furthermore the box had to be kept in a locked room with strict access controls. Anyone who had access to the room had to be under draconian NDA that included severe personal penalties for leaking anything (like enough to financially ruin you). Also everything was watermarked so you couldn't release photos without them knowing exactly where it came from.
The box was taken back and replaced with a standard consumer model after the launch. Most people in the company weren't even aware they were a launch partner until after the announcement.