Lead has a quantifiable effect that directly infringes on the rights of others -- eating junk food does not. You cannot say that my choice to get fat infringes on anyone's freedoms more than you are infringing on my freedom by preventing me from eating what I choose. It's not a characteristic of a free society. Pure democracy is mob rule. We might as well ban gay marriage, too, because there are more people who don't like it and claim it infringes on their rights than there are people who want to get gay marriages (see how that can be twisted the other way?)