It's probably because the vCenters are in the linked mode group. the remote vCenters will have to be detached from the linked mode group, before you can downgrade the license.
I'm not sure what you were told, but this is in the vSphere 5.1 documentation center: VMware vSphere 5.1
To join a Linked Mode group the vCenter Server must be in evaluation mode or licensed as a Standard edition. vCenter Server Foundation and vCenter Server Essentials editions do not support Linked Mode.