So the reason we don’t quite consider it to be murderbone is because it’s specifically used to convert people. By converting them to the side of the antag, they get to stay in the round and are even technically their same character once converted.
The reason we don’t allow murderbone normally is that it’s not particularly fun for people to just be killed and forced to just spectate. Conversion is generally allowed because it keeps people playing, which is a lot more interesting and fun than just being dead.