Some places in the world a lawsuit for defamation by a casino owner might well be laughed out of court if the issue was whether or not prostitutes were on the premises. My goodness, most of these places have professional gamblers (not that they want them ~ particularly if they are winners) and even criminals trying to cheat them.
I've never considered casino ownership to be one of those desirable occupations anyway.
I admit I can't remember a similar action from a public political figure, and Adelson is a political figure even if the other side did the defining.
That said, as to truth, the NJDC has apologized and withdrawn the charge, which in my common sense, not legal, mind might well settle the issue. Were it likely true they'd stick to their guns. As to malice which I understand is the big hurdle, I understand someone from the NJDC, I believe in the context of the apology, acknowledged that had no real knowledge, like Harry Reid's informer, simply went to google and searched for scandal, which they found and repeated. To my non legal mind, that's pretty close to malice.
Will be interesting to watch. I don't particularly like legal action in political campaigns, I think duels were a better solution to certain charges, but I'm glad he called them on it, regardless of how it turns out. Maybe people will think before making false charges, unless they have the privledge to speak from the Senate floor.
As to your representation of casino owners, while I don't disagree, I'd note that charges like abetting prostitution, don't know if it's legal in Macau, can be quite serious due the level and nature of government regulation.