While I don't know if this is the official answer, but you can see in the XML code for firewall rules that the objectID is that of the ID listed in the Managed Object Browser. The objectIDs vm-25 in vCenter1 can reference a different VM than what objectID vm-25 may reference in vCenter2. I suspect this is the rationalle for the 1:1 relationship.
I would view the 1:1 relationship as more of a design requirement rather than a limitation as. NSX 6.2 bring to the table the Cross-vCenter deployment topology. This allows you to bring together multiple vCenter/NSX Manager instances to be managed centrally. The cross-vCenter design allows for one primary NSX manager and up to seven secondary NSX managers, each tied to their own vCenter.