I don't think your example refutes the fairness heuristic at all.

The first case is completely fair because anybody else could have done the same thing without any special access required.

The second case is unfair because you had to work at the company to get access.

>> If a company accidentally leaves a press release for a merger publicly available, I happen to guess the URL, and then I trade on it: Unfair (I have access to insider information that other market participants do not) but legal!

Not necessarily. Just because you accidentally left your S3 bucket open and I brute force my way to the link by guessing doesn’t make it legal. It can still be insider information. Insider information is not limited to people who have a duty to the company. If I break into the companies office and steal information and trade on it then it can be insider trading.

Your comment seems to imply that trading based on material non-public information in prediction markets is always okay, which is not the case. The CFTC just made a press release detailing some instances of invalid use of nonpublic information on prediction markets: https://www.cftc.gov/PressRoom/PressReleases/9185-26

Interestingly, the CFTC objects to a political candidate trading on their own candidacy on the grounds that it is fraudulent. So it looks like they could attempt to regulate self-trading quite strictly, at least if that theory holds up after a court challenge.

> I happen to guess the URL, and then I trade on it: Unfair

I can argue it is fair - anybody can try guessing the url, you don't have to be an insider to guess it

> I have a duty to the company not to trade on it.

To the company? Or to the stock market, as a participant in it?