I have to say that my reasons for trying to get my kids to eat less junk and more healthy foods may be unique. Many adult diseases begin in childhood. The fact that we are beginning to see formerly adult diseases in progressively younger children such as hypertension, hypercholesterolemia, metabolic syndrome and diabetes really started making me think about whether or not I was doing my kids any favor by letting them eat whatever they pleased now and worrying about the consequences later. Eating habits are established in childhood and as many of us have found, are very difficult to change once you become an adult; the longer you've been doing something one way, the harder it is to change and do it another way even when we know that the habits that we are trying to break are bad ones. Add to that mix, inherited genetic tendencies. Heart disease and diabetes are very prevalent in my husband's side of the family, and diabetes on mine (I'm diabetic). Daughters of diabetics have a higher risk of developing the disease themselves.
I don't feel that we are obsessed with food in my home. In fact, we don't discuss it a lot anymore; they know what are healthy foods and what aren't. Not that there haven't been opportunities for education, either. When my blood sugar got out of control a few years ago, I had to really increase the number of times a day that I was testing my blood sugar and they had never seen me do that before since I usually tested before they got up or after they had gone to bed.
It led to a very good discussion about how poor choices about what I ate had resulted in my having diabetes (and no, mommy wasn't going to die!) and how making good choices about what you eat can keep your body healthy even when you haven't done so good in the past.
It's true that what they do when they become adults is up to them. The best that I can do is to try and educate them and instill healthy eating habits now and give them the best foundation possible to build on.