It comes down to how much money they want to spend and accessibility. I see he has a security detail on his videos as they tell the speakers to take their hands out of their pockets and to put objects down to prevent handgun shootings or acid attacks etc. Protection against rifles is difficult as large areas have to be covered, 300 yard radius for most attacks or 600 yards if they really want to take him out. The drone idea is good as it can serve as a deterrence and is relatively cheap. But many places ban drones. You could put up bullet proof glass, have the event indoors, metal detectors, etc but now the event isn't as accessible and more expensive.
If the threat level is high, then you need more all encompasing security to check for bombs, 24 hour security, security assessments, specialists, etc and the cost is really expensive.