People misunderstand the word diet all of the time on food labels. When a soda has the word diet next to it they assume that it's okay to drink, when in reality drinking that diet soda stimulates more fat growth than normal. The problem is people don't know what's healthy or what's unhealthy. It could also be that they just don't care and want their unhealthy diet food.
The reason people become fat is either they are really lazy or they do not have the time on their hands to lose the weight. The chance of someone not having the right amount of time is very low, so basically fat people are just too lazy to care about what they eat and to exercise the weight off. This is coming from a current fat person as well, I am very lazy.
The main reason that people become fat is that their caloric intake is off balance. They slowly consume more calories than they expend in a day. The extra calories add up over time. Since the weight gain is so gradual, they don't notice it easily. Then when they do realize it, it's difficult for most people to have the discipline to drop off the extra weight.
Saying that certain attempts at diet make people fat is a way of justifying the failure of so many people. If you spend a long time (months) eating fewer calories then you burn, you will lose weight. There are other factors, and there are a huge range of ways to do it, but it will work.
No, dieting is not a cause of obesity. There are many other factors that cause obesity. One major cause is genetic, or other inherent health-related reasons. Another cause of obesity relates to one's lifestyle choices, particularly activity levels and eating habits. If one is on a diet, one does not necessarily become fat, although dieting can lead to so-called "yo-yo" weight loss and gain.