It's not exactly normal for South Africa to score so quickly. I think there's a combination of factors leading to that. For a start, when the wicket dries out, the pace means there's a lot of value for shots. We've seen that time and time again. Warner last year against India, Gilchrist and Clarke in the Ashes 06/07, Gayle 09/10, Viv Richards in 1988, Roy Fredericks against Lillee and Thomson back in 1975. Hayden's 380. And let's not forget that record run chase last time South Africa toured.
The loose ball does warrant a feature, but more so, what you've got here is a batsman coming to the crease and deciding he's going to play shots off the back foot. Amla made runs in Brisbane, but he barely played any shots to the seamers, much less off the back foot. Perhaps the impetus was there to take the game on, perhaps he just felt really good in the middle. Whatever the case, he has used the punch and the flick to knock the bowlers off a reasonable length without necessarily taking them to the fence; that comes when they start searching for a good ball.
Starc seemed the most ill at ease and chaotic; he was really what contributed to Greame Smith's scoring rate, producing that rarest of things for Smith, runs through cover and point. On the other hand, Johnson demanded the most respect and he was bowling almost exclusively short balls. To some extent it was more width than length, but by taking on the straight ball, the bowlers were forced wider. You could argue that they didn't try to york anybody, but it's not like they were going to bowl a few overs of that.
I guess the simplest analysis is that there was mainly life with the new ball, with the ball swinging around for a few hours being enough to cut through the middle order. Now the ball doesn't seem like it will favour the new ball specialists as much, more favoring bowlers who hit the deck.