The OEM documentation reveals that,
This topic shows how Mobile Operators (MOs) can enable and test Windows Phone updates to work with zero-rating. Zero-rating allows users to download data, such as device updates, without being charged for data usage.
Add the following two URLs to your network as zero-rated sites:
Once those URLs are zero-rated, there are no more actions to take to get zero rating in place. It will be effective for all Windows Phone devices (both operator variant and open market devices). The first eligible update for zero rating is Windows Phone 8.1 GDR1.
Good to see that, it is valid for both unlocked and operator variant devices, if enabled by operator.