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The ones at the start of the machine are there as guides, to make sure bags are on the conveyor belt and not rubbing against the surfaces inside. The ones at the exit are to keep impatient travelers from reaching into the machine to grab their bags (because then they would get slight exposure to radiation).
The machines themselves emit very little radiation, as that could be a occupation health issue otherwise since there are TSA agents stationed right by the mouth and exit of the machine.
Source of info: just asked a buddy who works at the local airport.