I don't understand how volunteer, non-sworn officer, non nevada licensed security guards (volunteer "goons", who are just private citizens at a private event in a public convention center) can use physical force to take a non-violent trespasser off stage without exposing themselves personally and their organization to substantial liability. Not super familiar with Nevada law.
(Obviously doing crowd control, providing information, and front line emergency response is absolutely fine, although tbh even that they should probably have guard cards in most jurisdictions for liability reasons. If someone is violently disruptive, as a private citizen go for it, but unauthorized speaking on stage is pretty far from that. Would be hilarious if dude makes more money from that than he was stiffed by his employer.)
I doubt this is anywhere even close to the frontier of physical confrontation DEF CON goons have been involved in in the last 10 years; it's a drunk, druggy conference. I imagine at this point they know what the boundaries are. And if you're going to use words like "non-sworn", let's acknowledge that Dmitry was surely better off being escorted out by the Goons than by actual Vegas security.
The USA has fairly robust personal property laws. Each state is a little different, but for the most part, you have a right to defend yourself and your property, including removing trespassers from your property.
I'm not trying to defend DEF CON's overall actions here, but in this case it looks like the physical interaction itself was essentially consensual? The goons asked Dmitry to leave, his response indicated that he wasn't going to leave voluntarily on his own but also that he would cooperate with a minimal physical display that demonstrated the non-voluntary dynamic. I suspect that if he had physically resisted or even just gone limp, they would have escalated to actual security guards, who then may have laid actual criminal trespassing charges (if for nothing else than to cover themselves).
(Obviously doing crowd control, providing information, and front line emergency response is absolutely fine, although tbh even that they should probably have guard cards in most jurisdictions for liability reasons. If someone is violently disruptive, as a private citizen go for it, but unauthorized speaking on stage is pretty far from that. Would be hilarious if dude makes more money from that than he was stiffed by his employer.)