The problem with using grams as the primary unit of measure is that in a country which uses imperial measurements for most things, it's actually quite vague to the general public. Do they even know what a gram is? Can they visualize it, or is it just some mysterious unit of measure that they have no idea what it looks like? What does a gram of sugar look like anyway? Other than scientists (who deal with metric measurements) those living in an imperial measurement country wouldn't be likely to be able to describe what the volume of one gram of sugar looks like.

But a teaspoon isn't quite as vague sounding to someone who lives in a country which uses imperial measurements. Granted, when they think of teaspoons, they're probably thinking about how many heaping spoonfuls of sugar they scooped from the sugar bowl into their coffee, instead of a measuring teaspoon (which is much smaller - probably closer to what fits in a demi-tasse spoon, instead of a sugar shell spoon), but at least speaking of sugar content in terms of teaspoons gives thos who have no idea what a gram of sugar looks like, some concept of just how much sugar there really is in that candy bar they managed to scarf down in 3 minutes.