Well, the same reason US troops in UNFOR respond to Norweigian, French, German and generals of other nations that are in the UN Forces military structure.
It's generally implied that because there are a lot of nations involved in this, that therefore any body in it is in that same rank structure.

I see, but that doesn't make any sense. If they work for the UN, why would the Brit general cut them off near the end and left 'em to fend for themselves?
I fully admit I might have missed something. I was often too blinded by the unapologetic glam and the 80sness of it.

It's supposed to be a United Nations quick strike military that responds to threats by rogue nations the world over.
Tax dollars from a variety of countries finance them.
This is actually in the opening paragraph.