I’m part of NYPD. I was told today that due to the riots all officers must report 7 days a week. I can refuse and risk various levels of suspension, or I can stay nearby and limit movement, etc. only acting in cases of pikuach nefesh.
Under the current circumstances, they can stay and report, however they should make the greatest effort to be assigned to work which does not involve chilul Shabbos, and as you mentioned stay nearby and limit movement, etc. only acting in cases of pikuach nefesh. This is because without the proper protection and police presence, the protesters and rioters could lead to actual סכנת נפשות רח”ל.
However if there is a specific daily pay for Shabbos work, they should donate it to tzedaka.
PS. I find it hard to believe they wouldn’t be able to get out of working on shabbos with proper hishtadlus through some of our councilmen.
See: Shulchan Aruch Admor Hazaken Orach Chayim 329:6-8.