With Warne I have to concede that given how absolutely stupid he is in regards to just about everything except playing cricket, it IS possible he thought he was just taking a diet pill. Particularly given his well publicised vanity and sensitivity given his physical appearance (ie. storming out of a press conference when a reporter made a fat joke about him, going to Advanced Hair, etc). Of all the rubbish excuses thrown up, its one of the more plausible, simply because its stupidity accords with what we know of Shane's off-the-field style.
That said, I don't really care why he took it, he took it. He deserved everything he got and probably more. If he's ever caught again, he deserves a life time ban.