Because the plane is landing on the water and not damaging anything when it does so, unlike trucks driving through a bush.
I have to say that I agree with the closures, but it should not be accessable by plain, train, or anything else for that matter. If you can get there without a motor, do it and have a great time when you get there. The way this is set out it is rewarding those with more money, which is crap. It's crown land, our land, use it, but keep it protected and as beautiful as it is.