> Any role of speculation in "running up prices" is absolutely
> offset by an identical role of speculation in "running down
> prices."
That's not necessarily true. Higher prices can become acceptable and normal, after the shorts have covered; higher futures prices can get "baked in" by producers locking in a price. Those prices get passed along to the products that are made from the commodities. For a speculator, there's no time limit: he can always roll forward into the next contract[*].
Also, as was the case in 2008, when you eventually see the correction in the market, you've left behind a trail of destruction. People eat every day, production of food is a series, not an average. In the same way that 9 women cannot make a baby in 1 month, statistics don't tell the whole story with food.
Of course: some farmers may temporarily take advantage of higher spot prices; some food producers may temporarily be hurt if they miss a hedge. The identical/offset concept does nominally apply to speculators in the long run, but if they dominate the market, there can be collateral damage to the natural participants in the physical market along the way.
Position limits are of course at least part of an answer to this (and there are plenty of them out there); I haven't looked closely enough at the current round of proposals to see if they would help to the extent that some of these people would like; but: it's not as simple as you say.
/jordan
[*] Some people might not understand this concept. Futures contracts have expiration dates where you "settle up" -- if you're a natural producer that's the day you deliver your commodity to the other side who is going to consume it in some way. If you are a speculator and you are long that contract, in the days leading up to the expiration of the contract, you can simply sell your position and buy the next month's contract. There's a difference in price, because the two contracts are fundamentally different things, but over time rolling a contract is roughly equivalent (certainly enough for this conversation) to having purchased a much later dated contract (say a year from now).