well correct me if I'm wrong but the fact that the legs are attached at the hips means it's always going to be loaded and a lever. I think you mean the back is taken out of the movement?
notice that platz is "leaning forward" ie. sitting back in the front squat pic? The knee alignment is about the same.
you can fuck your knees up with back squats too.
If your spine is perpendicular to the ground, there's no lever. Only when there's an angle does hip joint become a lever.
In the Platz back squat pic, it's mainly his glutes that will get him out the hole until his quads take over. The lower back is mainly a stabilizer and only becomes an issue if you lean over too far and turn the exercise into a hybrid Squat/Good Morning.
The knee issues from back squats are primarily as result of bad form and too much weight. Even with solid form, there's a limit to what the body can handle.
I just think back squats are a more natural movement than fronts.