Sure, using grams is great for comparison... But getting the average person to find out how many grams of xxx they just consumed, unless it is easily quantifiable as "all/half/quarter of the total amount" is pretty hard.
1) Calorific density is much more important then calories per portion. If the majority of your food intake is in the 300-500kcal/100g range then you are going to feel hungry more often, over consume and gain weight - the body's satiation feedback loop is calibrated for fresh, natural food (50-200kcal/100g) not doritos (500kcal+). Interestingly, natural foods outside the typical range are difficult to consume in large quantities (try eating 2000kcal of avocados or honey in one sitting). There's a good discussion of these issues in the Volumetrics book.
2) US "serving sizes" are completely artificial, to the point of being downright misleading. The food industry is essentially lying to consumers. Calorie density can't be spun.
So really, all you should check for is the fiber content.
(And it's physically impossible to make a cereal with more energy per 100g than butter.)
To a good approximation ALL foods have about the same amount of calories per 100g. The only real difference is how much water. (And how much fat, but in normal food that doesn't vary so very much from food to food. Water varies a lot.)
This system just encourages manufacturers to add water, (and salt) to their food.
Much better would be "fiber per 100 [kilo]calories", "percent of calories from protein", and "percent - NOT number - of calories from fat".