I assume you use the term “junk food” to refer to foods that are high in fat, sugar and/or salt, which are also not nutrient dense. Doritos corn chips could be an example.
A 1 ounce serving of Doritos is about 11 chips. How many people do you know who would count out 11 chips and stop eating them after that, especially if consuming them from the snack sized bag pictured here?
For 320 calories, you get 16 grams of fat. Fat provides 9 calories per gram, so this means 45% of the calories in the bag are from fat, providing about one quarter of your daily fat intake (assuming a caloric intake of 2000 calories) in 22 chips.
The term “moderation” is a bit vague. I happen to like Doritos. I might eat them twice a year. For me, this is moderation. Someone else might think moderation means twice per week.
Finally, think about this: if you were to beat your head against a brick wall 10 times instead of 20, you would say it was less painful, not more enjoyable. In the same way, moderate consumption makes eating junk food less harmful, not healthy.