[The Squatter of Amon Rûdh]

Quite right, Kuruharan. Even if Sauron didn't consider himself rightfully a god-king (which he obviously does), lords do not announce themselves. The Mouth of Sauron is the Dark Lord's herald in the medieval sense, and he is accorded a herald's protection. His job is to save Sauron the trouble of stating his position to unimportant enemies.

Sending his herald instead of going in person to meet with Aragorn and his generals is a studied insult. It sends the message that here is "a contemptible little army", which is completely beneath his attention. That herald's abuse of his inviolability in insulting his master's enemies is therefore unnecessary, although it serves to emphasise just how hopeless the Dark Lord and his servants considered Aragorn's position.