It's currently quite popular for various drone resources to claim that, but it's blatantly false as a general statement. Federally there are fuzzy (court-interpreted) altitude restrictions basically any place where a person might be concerned about a drone. Many states, counties, and cities have similar policies, often with better wording, either explicitly or implicitly (e.g., via avoiding people and structures) giving an altitude limit. All that aside, the fact that many of the issues fall back to property ownership gives a wealth of common law on the topic, and that law is not in favor of the drone performing low-altitude passes over private property unless it's particularly unobtrusive (and often not even then).
Yes, there isn't a blanket minimum in the US for drones. The issue is murkier than you're stating though, and it usually falls much closer to the side of "drones can't do anything to upset property owners" than to the side of "drones can hover a foot off your lawn and can't be touched." Here's a smattering of examples, but there are thousands of other such laws or cases if you want to give your favorite intern something interesting to study.
- There are safety considerations prohibiting most delivery drones from, e.g., flying too close to a backyard barbeque [0]
- There is case law regarding the ability to use the airspace immediately above your property [1]
- There's more case law to that effect, interpreting the drone's actions as trespass [2]
- When interpreted as trespass, often there would still be no consequences (since you've only suffered negligible harm), but states frequently have privacy laws [3] giving the matter more teeth, and in that case explicitly calling out aircraft.
- Other states have such laws too, with Florida's applying to individuals but focusing on government overreach [4].
