It causes people to lack discipline in what they eat. That's really the issue here. Sugar isn't some poison and it is not a magical 10x the normal calorie. People who eat a lot of sugar are usually still eating a lot of everything else. All those calories add up and lead to weight problems.

The first thing to losing weight is to get serious about how much you are eating. Write it all down. People tell me all the time they don't understand why they are gaining weight, and then we go through a day of eating. It's not just sugar or fat or some other thing someone has said trying to sell a book, it is simply eating too much. Oftentimes people don't even realize it.

I had the opposite problem with I started weight lifting years ago. I simply couldn't gain weight, and thought it was impossible based on how much I thought I ate. When I started writing things down I realized I was barely breaking 2k calories/day if I was lucky. It took concerted effort to eat more, and results quickly followed. Now I'm older and am not as active, so I simply don't eat as much. My weight has stayed the same for a long time.