The concept of personal space and bodily integrity is different from culture to culture. It seems like in Korea it's fine to grasp a random person's wrist. Where I live, while you wouldn't likely get in trouble with the law just for that, it's certainly not good manners. It could be considered threatening, as an attempt to restrict a person's freedom, of course taking the circumstances into account. If you grasp a stranger's wrist to save them from danger, or something, it's not bad manners, obviously.