Bribing employees to disclose confidential information entrusted to them is not kosher nor wholesome. I consider corporate insider trading on these markets to be analogous - if you're an employee and you trade, you are selling your employer's info for money. Nearly every employer would fire employees caught giving away confidential information for personal bribes.

In the stock market, Matt Levine likes to say that insider training is about theft, not fairness. You can be prosecuted for merely sharing info with a friend on a golf course who then proceeds to trade. Your crime is not trading (you didn't even trade), but misappropriating information you were entrusted with and not authorized to sell.

What about bets without insider participation, where you want the market to function as an aggregator of educated guesses? OP has one reaction to insider trading, but I imagine a very common alternative would be "those insiders make their money off of bettors like me, I shouldn't participate." Some questions are clearly insider-proof, but I imagine many questions have insiders who don't bet on Polymarket. If Polymarket is going to be a good prediction market, surely it should incentivize people to make predictions on those questions too?

Indeed. For those of us not gambling, it's really quite beneficial.

It's not that it's cheating _in the market_, but if people have an obligation to their employers, etc, to keep information confidential, then they are stealing from their employer by cashing in on it, as sure as if they had taken money from the till.