1. Its not really Hareidi, but merely frummer than a normal unit.
2. While this is hard to believe in the Hareidi world, many religious zionists are very frum – meaning frum enough to have conflicts if they were in regular units. Since the army is legally and politically committee to including Orthodox Jews in all units, and can’t admit that it fails to do so, they put the more frum of the religious zionists in units that are called “hareidi”. To call the “frum zionists” would be to admit that the army overall is not accomodating the religious zionists.