The link the OP provided specifically states in the headline "New York Governor David Paterson reiterated his position on Monday that a tax on sugary soft drinks is needed to curb obesity and recoup some of the billions of dollars a year spent on healthcare to treat people with diabetes and other diseases." Granted in the article itself it states Sweetened Soft Drinks and no mention of the word sugary.
That to me does not include diet varieties which don't contain "sugary" substances. Diet doesn't cause diabetes to my knowledge, but I know that the artificial sweeteners have been linked to causing cancer.