> his argument was that he never agreed not to trade on the confidential information.
Close, but not quite. Here's the TLDR of the case:
Mark owns a bunch of Mamma.com stock he bought on the open market.
The CEO of Mamma calls Mark, tells him the company is planning a new private offering of stock, with the hope that Mark would participate as a buyer and add more shares. However, Mark is not interested, probably because he is no longer confident about Mamma.com, and since the offering would dilute the value of the shares he already holds, he sells them immediately.
So Mark traded on inside information, yes. However, that does not make his action illegal. It would only be illegal if Mark had agreed to keep the information confidential.
It is important to note that Mark here is not an officer or similar in the company, and so he does not fit that definition of an insider. However, the legal theory is if someone who is an officer, such as the CEO, communicates with a third party like Mark with an agreement to keep the information confidential, then the fiduciary responsibility to keep the information confidential transfers to Mark. This would make Mark's trade illegal insider activity.
This was the crux of the case: the CEO says he told mark the conversation was confidential and that Mark agreed. Mark says he never agreed to any confidentiality. The Jury believed Mark at least to the standard of reasonable doubt.
The reason the confidentiality must be explicit is because otherwise bizarre and immoral situations could occur, such as overhearing a conversation at a restaurant making you legally responsible as an insider. Or a sociopathic/manipulative person could communicate information to a group in an on the record context creating a legal liability on the others present without their consent. Obviously that's not workable. If you've ever been on shareholder calls where it seems the executives are sticking tightly to a legal dept approved script, this is why.
If you're getting the feeling that insider trading cases are highly ambiguous and circumstantial, you'd be right. It's probably due for reform, but I won't pretend to know how to fix it. I would point out that insider information passing between third parties is a de-facto currency among professional traders, so enforcement does seem rather capricious.
In any case, I am not a lawyer, not your lawyer, and have no first hand information on this case other than what I've seen go by in the media.