[sigh]
Not every self-destructive behavior is an addiction. Not every obsession is an addiction.
Why does it matter to be precise. First of all, because so many people battle drug addiction, and others judge them without understanding, assume they can just say no and make better choices. The false equivalencies just add to the misperception. Because while there may be a few seriously disturbed people out there who would let themselves die watching a TV program--people who would deserve compassion and require mental health--almost everyone else could easily stop such an insane behavior, so if that's what passes for addiction in their mind, they don't see why people with addictions can't be more reasonable and stop. But nobody's laced a TV episode with addictive chemicals, then targeted people too young to know better, and hooked them physically regardless of what they want.
It also matters not to call everything an addiction because doing so is a way of shaming mildly unwise behavior by attaching a pejorative label. Some people aren't as active and don't eat as healthy as others. It's their choice, and the people who fat shame them and pat themselves on the back for being fit probably have something else in their lives they could improve. But calling sugar or TV an addiction just gives the fat shamers of the world more ammunition.
As a drug addict, I can tell you it's possible to say no and make better choices. Before you've taken any drugs, it's very easy. After the first time, it's still pretty easy. When you've taken drugs for a while it's hard. When you've taken them multiple times a day, everyday, for years, it's extremely difficult, but still possible. You hear stories about people going cold turkey and braving it out through willpower, which I would say is VERY hard, but even just hitting rock bottom and checking yourself into rehab, which is by no means impossible, is saying no and making better choices.
Also, there are different types of addictions. A drug addiction is a psychological addiction, like gambling, combined with a chemical dependence that strongly reinforces the psychological component while also creating physical symptoms.
Finally, being fat is not mildly unwise but one of the leading causes of death in the Western world by any measure. It is a choice, like you said, in that a majority (at least 51% but almost certainly much more than that) of obese people do not have a medical condition that forces them to be obese. They should make better choices, like drug addicts should, because being fat or a drug addict is bad for you (except that choosing to go to the gym once in a while is a LOT easier than choosing to check into rehab after hitting rock bottom).
Call it "shaming", if you will, but more than a third of my country (America) is clinically obese (which means doctors, not just 'shamers', are advising less fatness on their part) and another third is overweight and at risk of obesity. Almost everybody knows somebody that has died of obesity-related illness. It is no laughing matter. Telling people to be okay with themselves when they have no other choice but to be fat is one thing. But it's irresponsible to tell them to be okay with being fat if they COULD choose a healthy diet and exercise, and live longer, be more active, and be less prone to disease and injury.
I'm one of those who "probably have something else in their lives they could improve" but if my wife, brother, or someone else I cared about became fat even though they had plenty of money and free time to exercise and eat healthy, then I WOULD suggest that they exercise and eat healthy. The fact that I am a drug addict, and that I should try to improve myself, would have absolutely no bearing on the fact that someone else was fat and they should improve THEMSELVES. Two wrongs don't make a right; they make two separate wrongs, each of which still needs to be fixed on its own terms. And for many people, fatness is easier to fix than they think.
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