This isn't quite true. Cheap drones may not have this capability, but controlled, safe landings of quadrotors with a single failed motor have been demonstrated, see this ETH Zurich video from 2014:

https://www.youtube.com/watch?v=ek0FrCaogcs

Obviously there are still other single points of failure such as the battery or controller, but at this point there's no excuse for crash landing a commercial drone just because a single motor (or rotor) has failed.