At least I know the rule now.
Incidentally, at our game today, some of the opposing team's batters used our on deck circle. Late in the game, one of the players who came over actaully apologized to our head coach since she knew not everyone likes it when that happens.
I can't speculate as to why on deck batters aren't directed to stay in their own circles when this happens, unless the blues are trying to avoid being perceived as booger pickers (or as I've seen on the other board, an OOO).
As a sidebar, I had to be Varisty 1B coach today and we actually got win #1 today.