By nature horses don't really carry themselves the way we expect them to when we ride. You will see some horses collect and perform nicely when they are loose, but they are generally provoked and/or showing off.
In nature, the only way I see a horse really utilizing its assets and collecting itself is at a full gallop where its depending on its speed to escape a dangerous situation, and cannot afford to be lazy or clumsy (i.e. rely on the forehand), it will lift its front end up and utilize its engine (the rear end) to be as light and fleeting as possible in order to gain the most speed.
However if horses are leisurely traveling from one side of the herd to the other they are not worried about swiftness, and therefore probably are more on the forehand.
One way to relate it to people is slouching. Everybody slouches, the only time you sit up straight is when you have to, when you are exercising or playing a sport you (or should!) keep your shoulders up. However when your at home watching tv on the couch your slouching, and when you go to the fridge for some snacks or your hanging out with your friends and your comfortable, your not going to worry about if your posture is correct.
However if your running a race (or for your life, hopefully not) your back is straight up, your light on your toes.. yet if your in the mall running to catch up to some friends your not utilizing all of your energy and your posture probably isn't conducive of winning a race. Even the most out of shape person can run 20 yards to catch up to someone. (i.e. ME!!!!)
Now back to horses, I'm not saying wild horses are "out of shape" they are certainly equipped to travel far, but in general they do what your horses do in the field. Graze graze graze. They really only run if their playing or if their threatened, and even then they only run far enough to feel safe.
Like, when you go out to catch a rotten horse (like the way my gelding used to be) who doesn't want to be caught. You are the threat and the horse is enacting the flight response. He runs far enough to be a safe distance from you (the threat) but once your far enough away he stops to graze again. Generally, in the wild the threats will follow them, [like wolves, say] so horses will move a greater distance with a swifter speed, but they will stop as soon as that threat is out of their sight, smell, and hearing range.
Also, with feral herds, there is natural selection. Horses who have poor enough conformation will be eaten by predators. With people, horses with poor conformation are only susceptible to the ailments associated with that conformation and not the actual threat of death (at least not immediately). So theoretically, most of the horses in the herd have decent enough conformation (i.e. topline if need be) to survive the next day. A horse with really poor topline, may be the slowest horse and therefore the next one eaten.
The ponies, however have no natural predators on the island though, I thought. So they really just graze themselves to their content, and hope they aren't going to be wrangled up in the yearly sale.
So to answer your question, they have naturally built enough topline to keep them out of danger for the near future. (Except the ponies, that is a good question, I'm sure many of them are poorly built, nothing bad happens to them because they have no natural predators.. The mustangs are still susceptible to wolves and I'm sure Mountain Lions or other predators, as well as people who also wrangle them up and sell them)
Horses just perform according to instinct, if they feel they are in danger, they naturally will lift off their forehand and carry themselves and become more swift.
However domesticated horses, in general, will have much more well developed topline than wild horses because horses learn to carry themselves under saddle all the time, or else are asked to by their rider. Your horse at home doesn't necessarily need a developed topline for leisurely riding, but you should be asking him to carry himself correctly (aside from building topline you avoid the injuries that are associated with clumsiness, horses pull shoes more easily if they are just running through your hands and not executing their movement properly, and it shows a more sophisticated rider).. for the sake of proper riding. Some horses are more prone to carry themselves than others, usually the lazy-heavy mouthed forehands rely on their forehand, while the light mouthed, push button horses are more likely to carry themselves (or at least are more easily directed and will not fall out of frame as easily as the lazy ones). That is a bit a general, and it all depends on the type of bit you have and the type rider you are.
Hope that helped!