DEF CON bans social engineering expert Chris Hadnagy

That has definitely not been my experience.

I've seen it once where the person was an extreme, boundary pushing pest to the point where it was an open secret when joining the org that you had to learn how to structure your work there so as to minimize interactions with them. But they had name recognition outside the org and were careful not to push the wrong people too far so it was always one of those "no idea what mr mouse's problem is, mr owl has never done a thing to me" situations with leadership. Eventually they were pushed out, officially for a CoC violation that dozens of other people had done, but everyone knew that it was because he fucking sucked.

Another time I thought some sketchy CoC-as-excuse stuff was going down and while I didn't know the details, I thought I understood the dynamics at play, and defended the accused, because I knew him, and he had always been consistent and honest in our work together. Over the next months and years I pieced it together and the real reason he was leaving was heinous and the real reason it was covered up were PR and funding related. Decisions made by people higher up for their own reasons. I was kinda right about the coverup so but wrong about why. Supporting him through that because I assumed the CoC enforcement was unjust remains my greatest professional regret.

So I don't know, I'm sure you have reasons for using the heuristic you do, too. And there's definitely room to disagree about whether CoC action is the correct enforcement mechanism for these things or whatever. But while I don't find "where there's smoke there's fire" to be a very useful rule in general, it has seemed pretty consistent here.

When enforcing CoC violations it's sometimes necessary to withhold details to prevent retaliation against the reporter (when the circumstances would make it obvious to all who was involved), and to defend the org from liability (kicking someone from your org after the fact doesn't prevent the victim from suing your org)

Every conference since forever has maintained a "that person is an asshole, they aren't allowed to attend" list.

Prior to COCs the list was unofficial, the rules unwritten, and often the rejection masked (sorry "sold out" in the days before the internet so less likely you'd find out that was not true... or just drop your signup form & check in the shredder).

There's no way to prove anything. It's DEFCON, not a court of law. The organizers can refuse anyone for any reason. Either you trust them to make good decisions for the benefit of the conference and attendees... or you don't.

This is often true. It pretty much says, "We reserve the right to ban you for unknown reasons if it serves our purposes."

It's kind of embarrassing to see what's become of the hacker community. They used to be against arbitrary rules and power hierarchies. But they aren't the first revolutionaries that morphed into the people they railed against.

I'm definitely curious to see when/if they release information on what the bannable transgressions actually are.

I'm not American, but I do know that libel laws in the US would prevent this.

They did not witness whatever event this was, they are relying on witnesses.

Publishing those witness statements would be negative to the witnesses, the accused, and to defcon.

Basically you want everyone to put themselves in a compromising situation to satisfy your curiosity.

They have told him why he is banned. If he wants to share that information with you or the general public, he is free to do so.