This occurs in many traditions. As an example, Freemasonry uses the four directions in laying out the Lodges. The four directions are mentioned when opening the Lodge, but only in reference to where people sit. There is no evidence either way that the ancient Druids used Guardians of the Wiccan variety. ADF ceremonies are open to any people who won't disrupt the ritual.
Sacred space is open for entry and egress at all times.
We are an inclusive, not an exclusive organization. Part of ADF's using open rituals is based upon having sacred space. That is what this ritual is meant to create. To use physical world security analogies, a circle is a fence. A Wiccan circle is the equivalent of an eight foot barbed wire fence with four security guards.