In this case I don't think the secluded spot is the issue.
It's being able to look at a map and say; "there's 2 or 3 campsites in the general direction that I want to go, I'll just drive and see how far I get, then pull up and camp at the nearest site."
With the QLD system (and I'm guessing the SA system will be similar) that is impossible.
As krisandkev noted, the logic seams to be that we're expected to book a spot at all 3 sites and only camp at the one we can make it to.
The example I used earlier is in the area around Kenilworth, Jima and Imbil.
This area is absolutely packed full of 4wd tracks, you could literally spend weeks exploring around in there.
We throw our swags and an esky in the cars and hit the tracks, we just want to follow our nose and see where we end up.
Pretty much every track crosses a main trail at some stage, you might pop out at Imbil, or right over the other side at Kilkoy.
There is about 6 camping areas dotted around the area, but according to National Parks we have to plan exactly where we will end up and book before we leave home, because once you're on the tracks there is little or no phone coverage.
So apparently you're not allowed to say "lets just go down that track and see where we end up" any more......