I think they buy because they need to do something with their cash.
NAB also bought a huge chunk of ABC learning and are sitting on 50M loss.
you have to ask if CBA wanted to exit now how could they do it?
in Circumstances that if I wanted to sell 100,000 shares today I would have had to look at an average of below 50 cents I think if you have a 1,000,000 shares then you have a problem.