Because his actions were consistent with the ideology, at least as some viewed it.
The FTX failure was because they were extending leverage to customers to purchase and short extremely volitile cryptocurrencies, including obscure/illiquid/likely-worthless ones (which they marketed heavily). The fact that they also diverted or personally pocketed some hundreds of millions on the back of stimulant-mania induced exuberance at the value of the magic-bean collateral they were holding didn't help things, but it wasn't the proximal cause. If pilfering the company were the issue it looks like from the magnitudes disclosed so far they could have just operated out of it.
The reason it's easy to link to EA forum posts telling people to NOT scam for the "greater good" is because it's responding a common view: it's a natural conclusion of navel gazing utilitarian-consequentialist thinking, especially if amplified by viewing all ethical questions through the lens of extinction risk. Doubly so in the face of "rationalist" ideology which is prone to reject received conventional wisdom in favor of bespoke and often self-serving rationalizations. Ordinary non-utilitarian-consequentialist charity communities don't need regular reminders to not scam people.
"Is it infinitely good to do double-or-nothing coin flips forever? Well, sort of, because your upside is unbounded and your downside is bounded at your entire net worth. But most people don’t do this, because their utility is more like a function of their log wealth or something and they really don’t want to lose all of their money. (Of course those people are lame and not EAs; this blog endorses double-or-nothing coin flips and high leverage.)" -- Caroline Ellison CEO of Alameda Research, responding to Scott Alexander of SSC in Feb 2021
"take advantage of strategies other people are biased against using" -- Peter Singer, "The Most Good You Can Do"