They would have no reason to do so.
Not supporting existing customers is very different than not selling directly to new customers. There isn't a foolproof way to implement this that wouldn't risk affecting service to drones outside the US.
Doing something that dumb would incentivize people to reverse engineer the firmware. After Google shut off web service access to older Nest thermostats,
some people hacked the firmware so the Nest would call new web services to allow them to continue to work. The same thing could happen with DJI drones.
Along the same lines, if DJI really wanted to play hardball, they could provide updates to US firmware that removes the need to call home, allows RID to be disabled, and turns off other limits. They could say this is a research-only firmware, and DJI does not authorize its use in FAA-controlled airspace.