@treos
Its not exclusively for mapgen. Almost all matchmaker maps have rotational symmetry afaik. Or atleast not the regular symmetry where opposing slots have the exact same position on the map.
This is because maps with regular symmetry tend to have somewhat stale gameplay. Also maps with regular symmetry tend to look pretty bad and unnatural. See for yourself. Compare ten maps with regular symmetry versus rotational. You will notice that the former look relatively unnatural.
As someone who has spend quite a bit of effort in garnering feedback on maps, I also notice that maps with regular symmetry tend to get a lot of negative feedback, when played through the matchmakers. I suspect it is in large part due to this visual unattractiveness combined with the more stale gameplay element.