Every fat person I know drinks diet soda anyhow.
Then allow me to introduce you to me, a non fat person who prefers diet sodas. You may catch me sometimes consuming a ginormous meal and washing it down with a Diet Coke.
Also when traveling in the south, I amaze waiters by telling them I want unsweetened tea. I simply don't like the taste of sugary drinks.
All that being said, government has no business telling me what to drink or what not to drink.