It is a nice functionality to synchronize users. It would be better if we have an option to control the option.
1) User synchronization should not be happen if the sub-sites owner doesn't opt to do so(An option under user management).
2) Super admin should have control to choose whether the user synchronization occur on particular sub-sites or whole network.
In this way, the sub-site owner will not have the access all users whom they don't know.