The question is always what is unhealthy and in what quantity. So called "healthy" food can also become unhealthy in certain quantities.
That is just arguing for the sake of it. No one is talking about extremes or unusually high quantities. If we are talking a normal-sized meal for a person, they do get less nutrients and digestive problems (although not always) with fast food than they would if they ate something home-made (or food similar in quality). So for that person, at that meal, they are eating "junk".
If there are 2 burgers, one home made in a BBQ and another bought from McDonalds, the latter is "junk", because you don't get anywhere near the amount of nutrients that you get from the first. Even if the McDs burger does have 70 or 80% of the protein found in the first, it's not going to be as bioavailable, not to mention all the added chemicals it comes with.
There is a reason parents, for example, prefer their kids to eat vegetables and food prepared at home, as opposed to eating in Burger King and McDs all the time, and it's not just because they are stupid or ignorant.
Plus "junk" food in the fitness world also often refers to food that has negative impact on body composition. By definition, there is no such food, just combinations of foods in certain quantities which are better or worse for it.
I agree, although I wouldn't restrict the negative impact in the definition to just body composition. It's also about the quality of the food, the amount of processing it's had, the quantity of nutrients in it and so on. So you could technically eat a meal at McDs and stay within your caloric restriction for the day, which wouldn't negatively impact your body composition, but is still of much lower quality than alternatives such as home made food.