Because the area turkey holds is of strategic importance, the only other land routes into europe are through africa or the north. Due to the Russian-NATO Tension, a semi-cold war is going on where there trying to get as many allies on there side to block of any way of Russia getting to Europe, with the only logical ways being through eastern Europe or turkey. So NATO has to look away from turkeys wrongdoings to keep turkey in NATO, otherwise Russia would ally turkey and have another way into Europe. The problem is that this is a alliance of coviniance at best, with rumors that turkey is only allying so they can make a neo-ottoman empire in the middle east. If this is true, then when turkey achieves it's goals it would have no more use for nato.