The title makes it sound like Hamas is shooting Israel's bombs back at Israel, but Hamas' arsenal are primarily of Iranian origin. The article reports that they do scavenge explosives from unexploded ordinance, but not the amount. However, they do take large amounts of scrap like plumbing pipes from Israeli settlements, which are used to make bombs.
I'm not an explosives expert but I would hazard a guess that scavenging missiles and such that don't go off is not going to be a viable source of material to make more missiles, factoring in the risk involved.