The way i understand it hr usually requires a paper trail before you can fire someone but that paper trail requires the subject to break company policy to earn the reprimand that would make up that paper trail. If someone is just bad at their job and makes poor managerial decisions that isnt exactly a valid reason to fire them so its actually easier to promote them so at least they arent in your department, you can send them somewhere else in the company and make them someone elses problem if you cant find a reason to get rid of them.