It has to do with teams. If you start the map with even a single ally, regardless of whether that ally is a computer or not, you will start off in allies chat. If you have none, you will start off in all chat.
You can talk the observer in the Chat menu on the top, and just click the circle next to Observers. You can talk to them, but they can't talk to you. Even so, they can hear you on allied char (iirc), and in all chat.
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