The guy is a professor of nutrition and did this in a very calculated manner.
He didn't do it in order to to show the world that it's OK to eat shit! He indulged in some self experimentation (junk food included) and tracked his weight and health indicators along the way. I do the same stuff in my own diet regardless of my body composition goal--maintain calories while swapping food items. For example, two tablespoons of blue cheese dressing has a little more calories than a tablespoon of olive oil. Sometimes I just want some damn regular salad dressing even though it's not as great as flax and olive oil over a salad. Same goes for other isocaloric swaps I do.
Like Debra said, most people aren't going to be inclined to follow a diet consisting totally of junk and most aren't inclined to track calories to begin with.
And most people aren't eating small portions of bagels and other goodies "thinking it's healthy". Most people eat food without thought of it being healthy or unhealthy--they're just eating. I love the diner near my house.
I doubt as I look down the row of people at the counter I sit that all of them are sitting before their plate of bacon, scrambled eggs, home fries, and white toast with butter:
"Ya know, this meal is a bit too high in calories. Let's see about this meal--the eggs got some protein and fat in 'em; the bacon's got just a tiny amount of protein, but much of it is saturated fat and very calorically dense; I loaded up on the ketchup (almost half a cup's worth); and the white bread is just empty calories. Damn, this meal clocks in at 1500 calories - half of what I should consume in a day. Maybe next time I'll say no to the toast, keep the potatoes or swap with grits or oatmeal, and have an egg white omelet with some veggies and feta cheese."
Most people simply eat because food tastes good and they're hungry.