They would if that passenger filed a complaint of assault, which is what happened here. Whether that complaint could be substantiated by further investigation is a different matter, and the FAMs could have called all the witnesses they wanted to back up their statements.
I can speak with absolute certainty only for Italy (but I would be willing to bet not a small sum of money on the USA, UK etc being the same) and they WOULD NOT release the drunken disruptive passenger and arrest police officers operating on a foreign registered aircraft under request of the very plane's crew and seize their passports just because of "a witness or two" UNLESS there were immediate and patent evidence of severe misconduct on the part of the officers AND the physical injuries were serious. Assault, unless the physical injuries are serious AND evidence is deemed by a judge (Giudice per le Indagini Preliminari, "Judge for the Preliminary Investigation") "overwhelming" does not entail the restriction of the personal freedom of movement, in Italy.
Moreover, I would like to know the country which trusts the word of a private citizen over that of police officers... Usually they all tend to lean more or less EXCESSIVELY in favor of the police officers (and once again, I have found that to be the case even more in 3rd world countries).
That's a bit more difficult now that they have fled the country without waiting for due process.
FIGJAM_SEA, I'm not sure what your problem is with Brazil, but it's not "my" country.
I have got no particular problem with Brazil, my problem is with those who believe you usually get "due process" in the 3rd world...
Maybe you could have already understood where I'm coming from if you answered this:
"Just one of the many traits that I have found most "developing countries" around the world have in common despite vastly different geographic location and racial and cultural origins is the fact that when something like a mugging, a rape, a theft happens calling the police is NOT among the first things "the general public" does... Now you either tell me that Brazil is not like that (while in my experience it's a perfect example of that) or you explain us why it's so (which would itself explain why I wrote what I wrote...)."
I live and work here for a US company, much as I've done around the world for more than 35 years, so I have seen my share of crap countries. Brazil isn't the best or worst place I've lived, but neither is the USA. Each to their own.
Brazil isn't the worst one I have lived in either, far from it actually. The worst, by a very long shot, have been the Middle Eastern ones and (mostly for different reasons) the African ones.
In the many Asian ones and the 4 Latin American ones I have lived in I actually had a ball. That has not prevented me to see them for what they are. I would never vacation in any one of them with my wife and chlidren or take there my elderly parents...
Even in the best ones among them where it seems it's all fun and smiles and where many Westerners on 2 week holidays fall in love with the place and keep coming back with the rose colored glasses on (Thailand would be a great example of that) when something serious goes wrong you see the abyss between them and the West...