I found this much funnier than I should have.
Anyway, I think people hunt frost witches because their lore requires them to mingle with living people. They also have to eat these living people to survive. While theoretically it's possible to hunt in the outskirts of areas, in truth finding scattered roleplay like that is difficult.
Naturally, this makes them much more accessible than a small group of cultists or necromancers, that prowl around by themselves.
Evil characters and creatures themselves have always, ALWAYS been a target for people to kill. Any antagonist, any cultist, any necromancer, any witch has suffered a similar fate. I wouldn't say frosties are being singled out.