Well its not entirely out of the place decision not to pick a good pace attack so to deprive SA of good practice before 1st test and eat them undercooked at Gabba.
But there is another theory, when are you going to test these 4 against a TOP batting side? Only chance they would get is when 2-3 simultaneous injuries happen. Then basically you would be tossing a coin and picking 1 out of these 3-4. Better to have a look at them now and know what are your 1st choice options if couple of frontline bowlers break down, right ?