The difference in my mind is that prediction markets and gambling are betting on outcomes, not long term behaviors. You could make the argument that they are the same, in the sense that buying a stock is "betting" on a company to do well, but I think you'd be making a silly argument. Stocks are intangible these days, but they were traditionally a physical thing that one would trade. If you're betting on prediction markets, there's nothing to trade after the event happens, just payouts.