JimmyT said:
This may sound like splitting hairs but it’s not so much all owners collectively but each individual owner affected (which amounts to the same thing).
But isn’t that the point with someone taking over an area of common property, as discussed in the OP? Someone taking over common property affects everyone’s access to that area that used to be common property, but if the permission is granted no longer is. Thus I can see the reasoning for requiring unanimous approval.