It's not in the rules as written.
Because KCY-MOT isn't a published routing. I can understand AGR wanting to be firm on the rules in the case of an unpublished routing. Doubly so when there's a significantly shorter routing that isn't a circle trip.
ATN-CHI has one published routing. An exception to the circle trip rules makes sense here, since that's the only published route.
If it's not in the rules as written, how do you know what you write is true? I'm not even sure that "true" is the right word, since I'm not sure it matters what the correct answer is. Rather, it only matters what the AGR agent on the phone will do. We all know that different agents claim different things, and I do not see how you or I can credit one claim (the one
crescent2 got) as correct, and another (the one I received) as incorrect.
In any case, KCY-MOT was a published route when I booked it, and it was a published route when I was told it was illegal. It only isn't a published route now because Amtrak broke the Portland connection when it changed the Empire Builder schedule. It's exactly the same situation as
crescent2's (the only published route was a circle trip), but I got a very different response.