A fence behind the fence?

I've been amazed by videos of men and women casually climbing over our border fence.  How do they get up those twenty-foot steel slats?  They must be world-class athletes.

Another camera angle reveals the secret.  Immediately behind the steel fence (on the California side – the ocean is visible to the left) is another fence.  This one is chain link, and apparently, it's been securely fastened to the slats, because it's strong enough to provide toeholds and handholds for the climbers.

Who installed that chain link, and why?  Is California trying to keep Chihuahuas out or let illegals in?