Earlier quoted context omitted.
Simple. They... 1. Take steps that make it easier to identify leakers (like embedding secret numbers into promotional photos so they can tell who leaked them if they get published) and 2. They pursue anyone who dares leak information with the ferocity of a rabid dog. No real mystery here.
While I don't disagree with your observations, I think it would be remiss to exclude the fact that many Apple employees are incredibly loyal and believe in the company.
But the reality of working makes the magic unicorn dust wear off. I saw a bunch of prototypes and I knew a bunch of information that nobody else 'knew' but lots of outsiders suspected. I became more interested in my work than selling other people this idea that I had these secrets that I could pass onto others. (We were also told to 'eat our own dog-food' which makes me highly suspicious of every Software Update to this day. I always wait a couple weeks before installing anything new.)
That's an important part of the equation. If your employees are more interested in leaking information than working, maybe you're doing something wrong. (Just to clarify, I still consider Apple a great place to work; I left because I wanted to do something different, not because I felt that it was a poor place to work.)
[Edited for grammar]