I think most people don't really think or care much about what other people eat when they're alone or with their families, but it becomes an issue when we share food. Food is so closely connected to hospitality. Over here you can scarcely have a visitor, even a total stranger, in for more than a few minutes without offering him or her at the very least a cup of coffee.
And a lot of what people would call "garbage" (I do too, but affectionately ) is what other people consider the best they can offer. It's not supposed to be ultra-healthy, it's supposed to taste good, and the more luxurious and packed with cream, butter and sugar the "better" the host is. Gently pushing people to eat more is also considered polite.

It's not only when hosting a visitor, sharing food in any way is a way of connecting. I would be a bit disappointed if I invited a friend out to share a coffee and a gorgeous hot cinnamon roll or something with me, and all she wanted was an apple. Because I would have wanted to share the experience of eating something I really like. I would have no right to question or sneer at her diet choice, though. Just pointing out some of the reasons why people do react with their own feelings to other peoples diets.