EricF,
You are right, they are an animal that runs on instinct. However, if that great white wanted to consume the surfer, I guarantee that he would have. The femmoral artery, if severed, can cause somebody to bleed out and die from blood loss which is what I am guessing happened with that surfer. I think much of the problem that exists with surfers getting attacked at beaches is the fact that in-water visibility at a surf beach is very poor, and a shark's eye-sight isn't spectacular to begin with. Although a shark knows that you are there through his other senses, he can't sense exactly what you are. That is where the attack happens, he takes a bite (which can obviously cause you to lose a limb), realises you aren't a food source and he swims away. If you know anything about fishing, it is kind of the same reason why you use lighter and clear line in clear/calm water - so the fish cant see it.
I have swum with tiger, great white, blue and hammerhead sharks in the wild and not once have I ever felt scared for my life. I think the reason I have been comfortable swimming with big sharks is because it has always been out at sea, the water is crystal clear and the shark can see you pretty well, and can usualy tell that you are not food. I had an awesome experience swimming with a tiger shark once - I was snorkelling, and at one point I held my breath and sank down the water column to about 25 feet deep. As I did this, the big female tiger swam over me (about 12 feet long) with her mouth wide open, as if to show me her teeth, a moment I will never forget. I swam with her for about 6-7 minutes and it may sound strange, but I felt almost a bond with her. I didn't once feel threatened and could even swim along side her and give her a pat. It was a truly amazing experience I will never forget!!
You may think I am crazy, but I see these animals for what they really are and have been lucky enough to be able to study enough shark's behaviour that I know when it would be too dangerous to get in the water. For example, you should never get in the water when you are throwing bait/chum over the side, but if you are going to do that, make sure you have shark protective chain-mail on or better yet, go with the pros and get in a cage lol.
Hope this helps shed some more light on a truly misunderstood animal that I dearly love
Cheers,
Jordan