Did not know that - thanks for the info.
Do they have to check in with air traffic control when flying in certain flight paths and altitudes?
You don’t need to be in contact with ATC unles you are in Class B or C airspace...which is at or above 18K (IIRC) feet (most piston single engine acft dont go there) or around major metro type airports. B requires clearance to enter, C requires 2-way communication.
All of the above is easily avoided.
I’ve flown from Oregon to Wisconsin 3 times without talking to ATC.