Uninteded Consequences of Mandatory Labeling

In light of the impending vote on Prop 37, which would require mandatory labeling of GE food in California, I found this post from David Henderson back in July about a previous mandatory labeling initiative in California to be quite telling:

I went on line a few days ago to order some vitamins. One of the items I ordered was Green Tea Complex. When I tried to place the order, I got a message in red saying that I couldn't order Green Tea Complex. So I deleted that item and the order went through. Today I went to the local GNC to buy the item I had bought many times before. It was on the shelf and so I picked up two. I told the salesman that I hadn't been able to order it on line. He explained that there is one Green Tea Complex for California and one for the other 49 states. I asked why. He said it was because the ones sold in the California have the Proposition 65 warning that the item contains ingredients that may cause cancer. (If I recall correctly, this Proposition, passed in 1986, was the first one I ever got to vote against, after I had become a U.S. citizen earlier that year.)
"Do you think the ingredients are any different?" I asked him. He answered that he didn't think so and that the only difference was probably the absence of the warning on the non-California bottle.