Ive been meaning to look into this. I had one last year that was still alive when I found it. Shot it about a half hour before last shooting time gave him about an hour then found him about 100 yard from the shot still alive and had to give him more time when I could have just finished the job. A gave him another hour and he was dead when I came back.
I suspect its not allowed because they fear people would take advantage of it and claim they were finishing animals off to get away with poaching.