I thought of a possible reason why they wouldn't do this. Basically, you've got two choices with the unbreakable vow ploy: Obtain a class of civil servants willing to give up their own magic to do the job (fat chance), or force the criminals to do it to each other. The natural answer is the latter right? Well yes, except the part where you have to hand a criminal whose crime was severe enough to warrant stripping some of his magic a wand and give him enough mental breathing room to perform a complicated, powerful ritual. Some of them are just gonna go along with it, sure, but you only gotta have one high-profile screwup before that kind of a policy is abolished.