might rule on US legality of Trade Sports and futures prediction markets

On I set a $5 bounty for the following question:

Is it against the law for US citizens or residents to trade on overseas futures markets such as

Many view these types of prediction markets as gambling or betting. Does anyone have any experience with the US legal system’s view of US residents and citizens participating in these sorts of prediction markets?

I has asked the question on MetaFilter earlier but got no authoritative answers. If you know the answer or know of some relevant cases stop by and pick up $5.