As for the "there's nothing wrong with sugar" preachers, because "a calorie is just a calorie" in their minds, let me ask this... are the nine calories from a gram of saturated fat or trans fat, the same as the nine calories from a gram of polyunsaturated fat or monosaturated fat? I mean don't we know that the body processes these different fats in different ways, some being good for the body and some being bad? Don't we know the amount of saturated fats should be limited in our diet, and trans fats should be avoided?
No one is suggesting that sugar should be avoided completely, nor should simple carbs. But these types of carbs are processed by the body in a substantially different way than other carbs like fiber, as well as proteins and fats. It's not normal consumption of them that makes us sick. It is excess consumption of them that does. It turns the body into a roller coaster of fat production, then hunger, then fat production, then hunger, then fat production, then hunger....
A person who wants to limit their diet to 2000 calories a day, but consume a lot of those 2000 calories as sugar and simple carbs will experience this roller coaster, and by the evening will have consumed 2000 calories for the day with their brain telling them "PLEASE EAT SOMETHING!!!!!". They will fail on this type of diet because limiting themselves to 2000 calories a day feels like a battle, one they may win for a few weeks, maybe even a few months, but eventually they will lose.
On the other hand a person who wants to limit their diet to 2000 calories a day, but consume very few of those 2000 calories as sugar and simple carbs will NOT experience that roller coaster. When they are ready to go to bed, they can frequently have some calories to spare in their 2000 allowance and not have experienced any hunger issues they had to fight. This diet can be sustained for a lifetime.
In regards to fruit, yes fruit has a lot of sugar, but it also has a lot of fiber, which slows down the body's absorption of the sugar. It will all be absorbed but over a longer period of time compared to the same amount of sugar coming from a soda. So fruit does not kickoff that roller coaster ride. However, if fruit is eaten in addition to a lot of sugar and carbs coming from processed foods, then the body is already on the roller coaster ride so the body's insulin spike just takes that sugar from fruit and turns it into fat too. The fruit didn't trigger the insulin spike, but it was converted by the insulin spike none the less. And for diabetics the fruit in addition to processed sugar and simple carbs just makes the blood sugar level go higher. If however a diabetic has a diet completely absent from other sources of sugar, they could probably eat many servings of fruit spread out through the day without problems.
The idea that a calorie is just a calorie is only true in a lab. Yes you will get the same amount of energy from any source of calorie when it's burned. But it's how the body responds to different carbs, different fats, different proteins is the problem Americans are suffering from, because there is a TON of sugar being added to our food supply by companies who want their product to taste better than their competition. And the zero calorie sweeteners also cause the body to have not so good responses, so that's not really a solution for the calorie counting crowd. Again, I'm not bashing the process of calorie counting. The amount of energy is simple math and unarguable. But when one ignores how the body responds to different kinds of calories, they will naturally eat the tasty calories, which eventually leads to them failing to limit calorie consumption at where it needs to be.