Only internal traffic within the country is free. For example, YouTube, which accounts for a large portion of global traffic, retrieves videos from a caching server within the country, so this traffic is free. This traffic does cost money, but in comparison to the revenue, it is insignificant.
However, if you enable a VPN, this traffic becomes international, and it is paid for by your provider or the backbone providers, including state providers. Look at the screenshot showing how backbone providers use ships and special equipment to lay cables across seas and oceans; this service is very expensive.
With a VPN, the costs for backbone traffic increase — this is traffic going outside the country and passing through the VPN.
Now you know how this leads to additional expenses for backbone traffic.
