RD
The reason the US doesn't allow non-US controllers is because all the radar facilities, whether approach/terminal or area/center, and most tower positions, are US government jobs either thru FAA or Dept. of Defense, both government agencies. All government jobs, by law, are to be filled by US citizens. Not necessarily fair but the law nevertheless. After 9/11 I believe it will be a long time before the US will seriously consider "privatizing" the radar ATC facilities and the busier (higher paying) towers. And the security clearance isn't the "given" it was before 9/11/2001. The US with a population of 300+ million citizens can easily staff the 15,000 or so FAA controller jobs plus Defense jobs.
I'm not sure but I believe the tower jobs that have been "contracted" out to the private sector take non-US applicants but are financially unattractive when compared the government positions and many foreign tower positions.