The traditional Blizz explanation for not perma-banning gold buyers was that it crsated more problems than it solved. They found that perma-bans lead to the guilty parties purchasing max level, BiS geared toons from RMT websites. Apparently, these accounts were stolen by the RMT sites from legitimate players, or, leveled on accounts which were paid for using stolen credit cards, which resulted in chargebacks from the credit card companies in addition to the credit card companies threatening Blizzard’s abilities to use their payment platforms.
The claim was made that substantial, but not permanent bans actually led to the players who had purchased botted/stolen gold to mend their ways and act appropriately.
Whether-or-not Blizz’s explanation was wholely true, I do believe there was method to their madness.
(I tried to find the blue post in question, but my Google Fu is weak today.