Good question with a political answer - that I don't know the ins and outs of. It was quite specifically stated to be issued to Aust and NZMilitary Forces. But several years later, Aust did relent in several cases......., said to have been by default or in error. But it did set the precedence that opened the door - but only if you got to hear about it - which I did from my RAR/RAEME colleagues! But like another example of a medal issued by the Brunei/Borneo state, the UK Military would not approve of the WEARING of the medal, presumably for political reasons (that the UK Military might well have been there, they were not there operationally). Nope, I didn't understand it either.
Anyone interested might like to see a talk given at the National Army Museum a few years ago, titled 'A JUNGLE TOO FAR. Britainand Vietnam'. Try you tube. I was asked to comment by the writer at a pre-talk meeting. Lo and behold, also present was one of the Tank Regiment Sergeants who was looking into the doctrine of using the big heavy Centurions in the jungle. They had already learned in Malaya that the Cents were too heavy to cross the bridges unless they were reinforced