You are missing a few points.
How are you sure that they got only 1 cold wallet? The 30k might have been sent from the deposit address to another one or to multiple or even at multiple transactions going to multiple cold wallets etc.
Even after gox people have to trade. If you want to trade big, you need lots of coins and fiat IN the exchange to do that.
this is Bitstamp cold storage address, which now reveal something interesting, these last transactions were made to refill the hot wallet so people who bought that 30K wall can withdraw their coins, but this is not the interesting thing, however there were only 148K
BTC in that wallet a couple of weeks back, so this means the owner of the wall already had his coins at stamp all the time and he didn't send them there like some guys suggested here.
what is interesting, or rather annoying is that even after Gox people still leave their coins at exchanges...
because I waited for hours and that transaction didn't move to any other address.
I do trade as well, and the only thing I keep on the exchange for open orders is Fiat, but I move coins in and out right away.