It's the other way around, let facebook build a separate, proprietary infrastructure for their service. I'm not sure if a real Private Network or a virtual PN would be the better comparison.
The government is not allowing that. They're banning differential pricing. Even if Facebook were to build their own network, they wouldn't be allowed to price discriminate based on websites visited.
Also, Facebook is not renting a government network, but a private network. That private network wants to partner with Facebook, but the government is stepping in and blocking it.
As it stands, the state, universities and other providers might have sponsored a lot of the infrastructure, while fb is just using it, so if they want their own, they should build it and only offer fb. for free.
If the state thinks it's a bad idea, they can not offer their own infrastructure to FB. That's not what happened. They decreed that nobody, even those who agree with FB, can rent their infrastructure to FB. That's the problem.
Also, Facebook is not renting a government network, but a private network. That private network wants to partner with Facebook, but the government is stepping in and blocking it.