It matters because there's a chance that this will go before an actual judge, who might rule that NYPD's counsel is wrong.
That internet service providers are not common carriers is just one example of an administrative FCC ruling.
Administrative rulings matter a great deal to the daily operation of government and how citizens interface with their services.
It has been and continues to be a major cost in terms of time, effort and resources for news organizations to drag every administrative ruling in front of a judge.
"NYPD's lead freedom of information counsel refused to release the department's freedom of information guides, citing attorney-client privilege."
In this context that is often the cause of a very serious inequity. The government has lawyers funded by the taxpayers arguing for the government's position against the public, but no taxpayer-funded lawyers are provided to represent the public against the government. Predictably the public interest then gets the short end of the stick in any case where there are no well-heeled private interests willing to pick up the tab.
The trouble is that the accountability system in government doesn't take that into account. The state's attorney is presumed to be representing the interests of the public, so nobody is provided to argue the contrary position when the state's attorney ignores his duty by advancing the interests of the state instead.
seems par for the course, given the URL.
But you raise an important and non-trivial point.