The nets are not continuous, rather they are staggered to allow fish etc in and out of the beaches, but they fish out large sharks. This reduction of the number of large (and therefore dangerous to swimmers and surfers), sharks from the local area is what prevents shark attack on the beaches, not isolating the beaches from everything.

You can visit their headquarters at Umhlanga Rocks, and take a daily tour. They have a rather good audio-visual presentation about how various sharks hunt, and how most shark attacks are mistaken identity, and accidental. They show a picture of a turtle from below, compared to a surfer paddling out, a snorkeler, compared to a seal etc.

They also have an awesome collection of things taken out of the stomachs of sharks they have caught.

The highlight of the tour is a narrated dissection of one of the sharks caught that day, with an explanation of their anatomy. An explanation of how their teeth are renewed, the Ampulae of Lorenzini (the electrical sensory apparatus in their snouts) and so on. One of the things I most remember is that sharks have a bi-lobed liver that can be up to a third of their live weight, so I can well see the attraction to a hungry Orca of a big great white.