They are either imcompetent or they just don't want to do it. The 556 barrel is threaded into the receiver just like many other weapons. It is somewhat like an AK variant more than an AR-15 (AR's use a barrel nut and the barrel itself has no threads. The 556 just threads into the receiver. I would say that they are copping out of threading it because they are afraid that you will add a non-compliant flash hider (OOOOOOO naughty!)
Find someone that will and have them pin your brake to make it legal...
Find someone that will and have them pin your brake to make it legal...