Eh, I don't think it needs that much over-explanation. We also commonly call the ROK "South Korea", and the ROC "Taiwan". They're both allies, but we still typically don't use their actual names in anything but formal settings. Going back to the Cold War, we used geographical terms for both East Germany and West Germany, which weren't in either of their names. I don't think this was out of a desire to dehumanize Germans, but rather because referring to a divided country as if it were not divided, using their official names "Federal Republic of Germany" and "German Democratic Republic" (which both claimed to be The Real Germany), is unwieldy bureaucratese.

(It's also not U.S.- or English-specific; you do the same thing in all these cases in Danish. Except for formal documents, it's Nordkorea and Sydkorea, not Den Demokratiske Folkerepublik Korea / DDFK or Republikken Korea / RK.)