You don't seem to understand things like changing the rules for institutional investors. Also read this article, which discusses one of the futures markets, food:
http://www.foreignpolicy.com/articles/2011/04/27/how_goldman_sachs_created_the_food_crisis
It touches on everything from the explosion of investment money to the effect it had on the hungry people in the world, putting hundreds of millions into hunger for parisite profit.
The GSCI is more of a index fund then derivative. Index fund can be used for long term investment purpose (therefore that long position only feature) especially when combined with equity for diversification since commodity doesn't have much correlation with equity market.
The impact of those commodity index fund to the commodity prices are still being debated. If there is a real impact, sure go ahead and get rid of them. But at least understand you are talking about fund there and we are still talking derivative market here.