In stock markets, insider trading is a big no not because it ruins someone's gambling habit, but because the entire concept of the corporation requires a certain amount of trust of financiers in financees. That whole pooling of capital thing, to do stuff that has too high a capital requirement to start individually. When shares are publicly traded, that trust is impossible when holders have to assume that they will be gamed by employee-owners and that would mean nobody outside the circle of those in the know would ever put in money and then you could just give up and declare that publicly traded corporations simply can't exist. "Don't bother investing, they will strip you".

Prediction markets don't have any "natural" reason like that for excluding insider trading. It's just "game designers" crying their hearts out when someone ruins their game by having an advantage.

The employee could not be an insider if his employer did not exists because of a lack of rules against him trading. The prediction market not existing would not make the insider any less of an insider (we are not tking about people inside the prediction market maker!)