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".
One of five possible definitions includes losing weight in no universe means that diet means "lose weight" -- you don't get to choose what Diet means on a can of soda.
Diet is nothing more than the things you eat, and a "diet" can be to lose, gain, maintain weight, or simply to pay attention to what you eat for the purposes of healthy eating. A diet soda is a soda modified to reduce/eliminate sugar, which again is in no way manifestly correlated with losing weight.
It plainly does, whether that usage makes you happy or not: http://dictionary.reference.com/browse/diet?s=t