Yes, management and administration is not a strong point of an DC agency, even the ones that people from every state vote for but have offices located in DC! Just think about key management, deployment and just plain operator error. Maybe keys can be deployed OTA, buy management of encryption adds a whole new level of issue onto the day to day things that are required.
Do not forget ALL the other agencies in DC that likely interop or at least may want to monitor the DCFD for situational awareness.
This is a partial list as I am just pulling this from immediate memory I am forgetting many agencies. DCPD, US Park, Capital Police, Federal Protective Service, Secrete Service, Arlington County, PG County, Montgomery County, Alexandria City, WMATA, WMAA and so forth. Remember DC owns the water in their area to the opposite shorelines.
As usual likely technical/operational decisions being made by non technical/operational management and/or committee because someone had egg on face and now that Encryption may be a no or low cost feature they figure it is the way to look better in the press.