Insider trading is at least regulated in the stock market, albeit imperfectly. Imagine how much worse it would be if C-levels could just short their own stock during a board meeting. No one without insider knowledge would touch it.

Matt Levine often says something like "insider trading is not about fairness, it's about theft." The problem isn't that it's less fair to some stock traders than others, and that stock trading should be some form of perfect gambling where everyone has an equal chance of success. Stock trading is inherently about exploiting information asymmetries — that is what all "non-insiders" are trying to do. But insider trading is wrong because it's effectively stealing confidential information from the company & shareholders, which is in violation of & conflict with the fiduciary responsibility that board members, executives, and employees generally have towards shareholders.

Conceptually, I think that is the right analogy to think about. Prediction markets "want" to be a more accurate source of information, just like stock markets, so from that lens "getting" information to be more accurate is good. When government officials are placing bets on prediction markets, though, it's a massive violation of operational security, and leaking confidential information. They probably think that they are acting anonymously, but it creates so many opportunities for unfriendly state actors to get information, especially if people do it consistently.