The old style Rusty's arms adjust the same way with no lock nut! They sell them separate, but they actually suggested loosening them off-road to allow the arm to twist.
With that said... They have changed the design to use a forged flex joint and a lock nut.
I can see why they changed. I am running the old style and if I don't keep them very well greased, they clunk. They have been this way for over 4 years now though with no big problems. Once I installed DBs, the clunking pretty much went away since the arms are not at such a steep angle, so I will live with them for now. I inspected them when I transfered them to the MJ, and the threads looked fine.
IIRC RC adjustable arms also use a threaded stud with no lock nut to allow twisting.
You would not want to run them this way with fine threads though, they will easily strip.
On another note, above 4.5" you need to do something with your uppers as well. Of course DBs or Longarms would be best!