I think it depends what you mean by obesity. Often, people base it solely on weight or BMI. By those metrics, healthy obesity is absolutely possible. If we're talking actual body fat content, then I think you start to have a point. With that said, I was told growing up and am a firm believer that health is a measure of how much energy you have going through an average day. That means a day of going up and down stairs, doing your job and traveling each way, and getting, say, 30-60 minutes of real exercise on top of that (meaning going for a walk/run, lifting weights, doing yoga, whatever you fancy). This also assumes no other factors such as high blood pressure, heart disease, etc. If all of those criteria are met, I don't give a shit what you look like.
I think the real point is you can't necessarily glance at someone and immediately tell the full state of their health. As for losing weight and keeping it off being easy for you, good for you. It's not that simple for everyone.
I am currently trying to drop an awful lot of weight and it fucking sucks. I want to do it now (mid 20s) while my metabolism is still on my side, but it is difficult. I work a 9-5 office job and when I get home I have to prepare dinner, do laundry, and take care of all the other little things around the house. It's tough to find time to exercise. I still do go for long walks on the weekends (try to get in at least 5 miles a day) and cook healthy using fresh ingredients and the like.
I still weigh around 311 lbs. I went through a period where I was doing things to drop the weight that were way less healthy than what I'm doing now (due in part to assholes who share your view). For a 4 month window I was working out 10 hours a week and taking in less than 4200 calories per week. Again, that's per WEEK. Did I drop some weight? Sure. I shit you not, I dropped around 50 lbs in that 4 month window. I went from 325lbs-275lbs and kept it off... for about a week. After that week, most of the weight went back on. That's a third of a year that was mostly wasted because I was more consumed with losing the weight than figuring out how to keep it off.
I said all that to say this: you don't know what people are doing. You don't know their lives. I would absolutely agree that when you see a person who's 400lbs in a motorized scooter at Wal-Mart buying the gallon tub of ice cream and a 3 boxes of Oreos, judging the fuck out of them is perfectly fair. When you just see a fat person, though, I think they deserve the benefit of the doubt. I don't think fat acceptance should be the end game. I'd love to be healthy and be of a normal weight and I'd love for as many people as possible to be there right along with me. It's a long, tough road, though. I think you comparing it to smoking is very apt. It's an ongoing process. You sometimes fall off the wagon. It's self-inflicted but difficult to stop permanently. For the same reasons you shouldn't just walk up to a person trying to quit and tell them to be stronger, stop whining, and give up the nicotine already, you can't do the same thing with fat people and food.