I believe a different attitude would exist if screeners are not required to repack. If I am unpacking at home and not having to worry about repacking, I don't take much care in unpacking. But if I have to repack after unpacking, I am careful about how and where I remove items.

In much the same way if a screener knows that he doesn't have to repack the suitcase, he probably won't be as careful in unpacking it. It's just human nature.