"More than seventy years ago, Congress made a choice to take that option away from local authorities. It conferred jurisdiction on the FCC and the state Public Utility Commissions to provide oversight, and gave everyone a federally protected right to access the phone network. That right applies to all phone networks, whether wireline or wireless."
I would put forward that it is because live-streaming on social media is not the same as using the phone line. When Facebook chose to shut down her account (which is within its rights) then she still had access to the phone line.
The federally protected right to access the phone network is unaffected. There is no federal requirement that would require Facebook to only shut down an account after receiving a court order.
