I read:
The DOT rule is that the airline has to offer (if purchased at least 7 days before departure) either a refund within 24 hours or a 24 hour hold without paying.
Does that rule apply to anyone, or only individuals living or purchasing the flight ticket from the US (or some other subset of humans)?
The 14 CFR 259.5 rule is for "Covered Carriers", which is defined as:
Covered carrier means a certificated carrier, a commuter carrier, or a foreign air carrier operating to, from or within the United States, conducting scheduled passenger service or public charter service with at least one aircraft having a designed seating capacity of 30 or more seats.
The 24 hours hold/refund requirement is codified in 14 CFR 259.5(b)(4):
Allowing reservations to be held at the quoted fare without payment, or cancelled without penalty, for at least twenty-four hours after the reservation is made if the reservation is made one week or more prior to a flight's departure;
It doesn't discuss any limitations on consumers, the Covered Carriers must provide the option to any consumer.
For foreign carriers, it only applies to the flights to and from the United States (as per 14 CFR 259.2).
Here's the DOT official guidance and the consumer information.
