Itemam E Hujjat
In surah Tawbah in the beginning of the Surah the polytheists were warned that they would be killed after the month of Hajj that is Muharram. This provided them a chance it becomes Muslims, so they did. The people of Makkah were not exempted, but they became Muslims after the conquest before the end of the month of Hajj. They were relieved from killing on the day of the conquest of Makkah but were not exempted from last verdict of annihilation.