There is zero terror involved here.
Terrorism does not have to involve an actual act. Just the threat is enough for terrorism to work. The threat here is that potentially an aircraft could hit a drone and crash, killing all on board. You do not need that to actually happen to determine the threat of the drones is a terrorism event. There's certainly no terror.....yet. Would you rather there was before classing this as an act of terrorism or is the potential not enough for you?