In order to be a citizen, one must be the owner by name of at least one parcel.
The rationale is technical rather than conceptual. The name on the parcel is currently the only way we have to ascertain ownership. As a policy, allowing group ownership as a prerequisite to citizenship would be impossible to determine... it would depend entirely on the individual's word.
Absent new land tools, citizens must be the owner of at least one parcel in name. The owner can "Set" it to the group (her name still shows as owner), but she can't "Deed" to group (where her name disappears).