This must be something relatively new. I was in the Marines from 1981-85 and I don't remember any non-citizens in the ranks. I too thought U.S. citizenship was a requirement in the armed services. Nevertheless, I guess its better to have immigrants join our armed services and earn their citizenship than to go on welfare.
I was in at the same time as you Sam and I had three Marines in my platoon that had their green cards. When things were exciting I had to tell my driver in Spanish which way to drive! More than half their paychecks went home to mom! Best form of foreign aid with the least graft this country has ever come up with!
I worked with a Gunny who coached our pistol team who came from Ghana and man could he shoot a pistol! I learned a lot. I never considered these men less than Marines!
Semper Fi!
TS