Suburban residential is very low-population-density, a few people per square mile, less during working hours. It wouldn't be hard to put a box next to your driveway where a copter could drop a package, and never get within a hundred feet of anybody.
The real population density issue is downtown, during working hours. Even then folks are on the sidewalk or indoors. There's nobody on the roof for instance. Copters could drop a package into a chute, again without getting close to anybody.
> Suburban residential is very low-population-density, a few people per square mile...
It sounds like you're confusing "suburban residential" with "backwater rural." Census data[1] shows that there are a lot more people per square mile than you suggest.
Even a small metropolitan area like Vernal, Utah, which has a population of just 25,224, has a population density of ~250 people per square mile.
The real population density issue is downtown, during working hours. Even then folks are on the sidewalk or indoors. There's nobody on the roof for instance. Copters could drop a package into a chute, again without getting close to anybody.