The parent doesn't seem to be arguing for NAT, just asking what sort of protections are in place. Verizon has done the correct thing here. No NAT, global addresses, firewall in place. That said, their firewall rules are very conservative, to the point where they provide NAT-like default deny of all unsolicited incoming traffic.

How ISP level firewalls should be configured is an interesting and open question for consumer level connections. For mobile connections default deny seems like an OK place to start, given that mobile connections tend to be shared and metered. I hope that this doesn't stifle the innovation that could come from true peer to peer mobile connectivity though.