In general, since they were forbidden from knowing which Night Action card they stole, the Thief's role has become somewhat pointless. So, he stole it, and what should he do? Use it on the person he stole it from? If it's a peaceful role (Doctor/Guardian), the Mafia will figure it out immediately and probably kill him. If he stole a Kill card, then everything is fine. But using the Night Action card on someone else doesn't make sense at all, because it could turn out that you kill a peaceful role. This is the worst-case scenario. Besides, several roles might visit that person, and then you won't know which Night Action card it was.
In general, since they were forbidden from knowing which Night Action card they stole, the Thief's role has become somewhat pointless. So, he stole it, and what should he do? Use it on the person he stole it from? If it's a peaceful role (Doctor/Guardian), the Mafia will figure it out immediately and probably kill him. If he stole a Kill card, then everything is fine. But using the Night Action card on someone else doesn't make sense at all, because it could turn out that you kill a peaceful role. This is the worst-case scenario. Besides, several roles might visit that person, and then you won't know which Night Action card it was.