How are you supposed to land from a jump? Stay in your 2pt until a stride after? Slip out of it and lean back when your horse is going down? What are you supposed to do when your horse likes to land on their face???
You want to maintain two point to the best of your abilities. If you get into the habit of sitting on your horse's back before landing you'll up your chances of taking down the hind rail of an oxer (actually, you'll almost guarantee doing it).
If you're having a hard time, it might be a sign that your leg isn't strong enough yet. Lots of two point on the flat should help correct that :).
The top photo is of an event rider over a drop fence; which is why you see the rider taking a somewhat defensive, safety position. Notice that the horse is happy, ears forward, that the rider has slipped the reins and is not interfering with the horse.
When I was new to eventing, and believed every riding situation could be handled from a classic forward seat, I attended a terrific clinc with the great Jimmy Wofford. During the lecture portion, I asked a politer version of "Why are eventer's positions so ugly?" and I got two answers from Mr. Wofford. First, "No Italian ever won the Grand National" (Italians - pioneers of the forward seat; Grand National = classic British steeplechase with horrific drop fences) and second, not taking the defensive position violated the first rule of equitation.
Dead silence among the clinic participants.
Mr. Wofford "You all know the first rule of equitation, right?"
Embarassed silence as a group of experienced riders scavenge their memory for the first rule of equitation. Heels down? Head up? Don't interfere with your horse?
Mr. Wofford "Don't fall off. Nobody can judge your equitation if you're sitting on the ground."
Staying in two point while negotiating a drop greatly increases the chance the the horse will misstep and send you over his ears.
The second photo is of a lovely, classic, correct forward seat rider.
I was taking a hunter/eq. Lesson and was told you should land with your weight in your heels. It felt REALLY good when I did. I have tried all of the different ways of landing (saying it two-point, leaning back but not putting my weight in the saddle, hitting the ground... etc.) and this one felt best to me.