Bets about singular events decades away are as far from pork bellies as you can get.

I don't see why that is. Political events are not tuned for maximal randomness, they are the results of many real-world trends and forces. Is a bet on China's GDP 10 years from now really 'as far from pork bellies as you can get'? There seems to be to be a vast wealth of theories and data one could, and people do, apply to such a question to do better than a coinflip. Are Hillary Clinton's prospects for winning the next presidential election really as fundamentally unpredictable now as the winners of the next Superbowl or Superball? I don't think it is.

Another reason they work as well as they do is that somebody does own something, whether's that's debt or insurance or agricultural futures or actual timber land.

You do own something at the end of it: a promise to pay from the losing party. This is as much as you own if you buy a bond or T-bill or mortgage, and the fact that you can't hold a bond the way you can hold a pork belly doesn't seem to stop people.

Another reason is that it's generally possible to make money without beating the other guy.

Subsidized prediction markets are also banned. There are reasons to use a PM even with a zero-sum payout, such as hedging or entertainment. And in practice, despite the zero-sum nature, existing prediction markets like Intrade did have users.