They weren't at a casino. They lost the money in trading markets. The same way they'd previously made a lot of money over several years, that being the very way they originally made their name and fortune.
That does not lessen the culpability of sneakily using other people's money for extracurricular purposes without their consent.
But if we're going to make "gambling" the focus, then let's call a spade a spade - trading-wise, Alameda did what Alameda has always done, since day 1. And a moment of due diligence would have so informed anyone considering investment in anything even remotely related to these people.