In particular, since ethics are about relationships, they are defined by the (sometimes unspoken) rules of that relationship. The ethical framework of a lawyer/client relationship is different than the ethical framework of two guys chatting at a bar, for example. If the lawyer hears a confession to a serious crime, it would be unethical to report it, while a bar buddy would be ethically clear to report the same crime. I would even argue that a lawyer reporting a confession to the authorities could be considered to be a moral (if unethical) decision, at least in some belief systems, though probably not in all.
And in this case...I'm not a marketer, but the fact that the other company was "furious" strongly indicates to me that it probably was an ethical lapse. When you're playing chess and someone makes a really key move, you may be disappointed, but you don't get mad. If they take your queen from the board when you weren't looking, you do get mad. The latter is certainly "outside of the box", but it's against the rules. And yes, there DO exist "sore losers" who will complain even when they have no right to, but I do think it was at least a minor ethical violation here, at least between him and the conference organizers who typically charge to distribute fliers like that.
[1] http://www.argee.net/Thrawn%20Rickle/Thrawn%20Rickle%2046.ht...
[2] http://everyday-ethics.org/2008/11/ethics-vs-morals-not-as-e...