Super nitpicky: sugar is 4 kcal/gram, but so is protein, and fats is 9 kcal/g, over double the caloric content.
By saying sugar is "pure calories," I guess you mean that it doesn't contain any fibre or micronutrients that might give it redeeming qualities (besides its taste), which is true.
Most people are not putting spoonfuls of beef in their coffee.
Sugar is, quite literally, pure carbohydrates. Most sources of protein are not, unless you're consuming refined amino acids (pro tip: they taste awful).
