Originally Posted by
SkyTeem
It's called "managed inclusion" - they don't claim that it's random at all. If your PNR meets certain undisclosed criteria, which TSA/DHS changes at random intervals, you'll be given precheck for a given flight. Managed Inclusion has nothing to do with citizenship since it's not mandatory (nor is it even collected in the APIS data unless the traveler volunteers it) that a passenger discloses citizenship when traveling on a domestic itinerary.
In my experience, they also use it to load balance the security lines during times when some algorithm predicts line wait times would be longer than desired. I've never gotten free Pre out of my home airport, where the security lines are never more than five minutes and about half the flyers have Pre, but I go on streaks where I'll get it more than 50% of the time from larger, more crowded US airports.