I can’t recall the case, but the ruling stems from an incident where officers were being set up to respond, and were ambushed, or required to go into situations where THEIR lives were in danger.
I would not make the case they should disregard their personal safety, or an operation they were already involved in that required their attention.
I would only suggest that if they were able to and took a pass, I would expect them to be held to account.
Certainly objecting to helping when they could, simply because a federal agent were taking down illegal aliens and came under fire, would be a gross dereliction of their duty.
Perhaps some loophole would give them cover, but I would consider that to be very misguided.