As far as I know, the US is the only country where sugar-free soda is referred to as "diet". The notion that it would help lose weight is totally new to me, I'm presuming that is because most countries outside the US would never allow it to be marketed that way.
It's called diet here in Canada as well. In any case, diet does not and has never meant "lose weight". If you pay any attention to what you consume, you are "on a diet".
It plainly does, whether that usage makes you happy or not: http://dictionary.reference.com/browse/diet?s=t