We have current methods to detect and protect against guns. We don't have current protections against say a drone diving into a crowd at a football game.
We have protection barriers to prevent vehicles from hitting large crowds of people. We have security procedures to prevent cellphone use bombs on crowds, but none that prevent an explosive-laden drone from wiping out hundreds instantly.
You might just as well ban crowded places. A drone solves the problem of getting a weapon to a target, which is relevant in a war zone but not in a public place.
If someone wants to bomb a crowded stadium, there are simpler ways than strapping a bomb to a drone