By 'fair', I mean 'all parties have access to the same information'. The stock market is supposed to give everyone the same information. Trading with privileged information (insider trading), is illegal. Publicly traded companies are required to file 10-Qs and 10-Ks. SEC rule 10b5-1 prohibits trading with material non-public information. There are measures and regulations in place to try to make the stock market fair. There are, by design, zero such measures with prediction markets. Insider trading improves the accuracy of prediction markets, which is their whole purpose to begin with.