I agree with CWnut & Woodland Mike and could also add the following theories.
During the battle if the soldier was wounded, he'd drop the belt, pull the buckle off and use the leather for a tourniquet or sling. Or during the battle, a retreat may be sounded and the soldier may drop the belt, leaving the buckle, cartridge box (box plate) and bullets. Afte the battle, a Confederate soldier comes along needed supplies, pulls the buckle and box plates off and keeps the leather, box and bullets. He doesn't want the US labeled stuff, but could use the box and bullets.
If a puppy paw broke off the back of the buckle, its pretty much useless. Pull it off, toss it in the trash pile and go to the supply wagon and get another.
Most are found with broken attachements.
Another theory I have is, these items were heavy, back filled with lead. A soldier who walked 10 to 15 miles in a day didn't want to carry any more weight than was absolustely necessary, perhaps they would lighten the load by removing the box plate, and belt plates.