NPS is in charge of the National Mall. They are heavily invested in the truth of statements about their work. Just as it was appropriate for intelligence officials to state they believed Russia hacked DNC servers, it is appropriate for the agency in charge to correct, lets be generous, misstatements about how many people attended an event they helped coordinate.
It is incredibly appropriate for the security services to state suspicion/evidence of foreign maleficence. (If it's true.. if it's a political suggestion.. they should be punished for propaganda) Although, Wikileaks has been nothing but consistent in stating it wasn't rush that gave them the leaks.
It is fine for NPS to report occurrences. However, it was not fine for them to make a comparison of inaugurations.
The offices of the government should and always should remain neutral. They should be judged on doing their job properly. Should the EPA pretend to be Milo on how they think that Trump's/Rep party's ignorance on Global Warming is bad? No they should be advising us on upcoming, and current threads, showing superfund cleanup efforts, creating effective change, etc.
It's hard to judge an organization correctly when it's trying to play politics. When you're wrestling with a pig, you're going to both get dirty, but in the end the politican likes it.
Comparing objective reality between two points in time is not partisan. It is factual. It is the truth. If the crowd had been larger than previous years, would you still object?
They were correcting a factually inaccurate statement made by the President of the United States, using evidence they were in possession of. One must hold truth above abstract concepts like "favor" and "taste".