People were freaked out when they realized that Niantic/Pokemon Go had been using phone cameras to create maps of the world, including the interiors of buildings, areas with dense foliage, private property, etc.

An army of drones that passes overhead several times a day could collect even more information—data—that could then be used in any number of ways, both benign and malicious.

It's especially concerning, because property rights extend upward, into the sky. At a certain point it becomes navigable airspace, but that's largely been a result of 1) difficulty proving when an aircraft crosses your property line and 2) necessity in facilitating air travel and shipping at ultra-high altitudes—a public good. Drones—especially commercialized, low-altitude drones—are viewed by many as entirely different.