This is regulated in part by state laws, but the lease agreement should spell this out. I would look over the agreement to see what it says about “Landlord right of entry.” If I were you I would fight to avoid having the lockbox put on. You can always file a landlord complaint.

I did a quick search for you… http://www.rentalprotectionagency.com/renter_rights.php#S The Rental Protection Agency shows that North Carolina has no statute dealing with entry.
That means, that yes, your landlord can have a lock box put on and enter at will. You should still be able to fight it through RPA Mediation. To start the process you will need to first file an apartment complaint.