Cop brain is very simple: You wither listen to the cop or you are a criminal. There are no shades of grey, and the US police have demonstrated this across the country this weekend.
"...to enforce federal criminal laws in the wake of protests..."
So not protestors, but criminals taking advantage of protests.
https://old.reddit.com/r/PublicFreakout/comments/gvmhh6/fbi_...
On one side, you have the generally disorganized public which naturally includes the opportunistic, the provocative, the desperate. On the other, you have the police and the military, nominally organized with a largely uniform mission and code of conduct.
Bad behavior by the government is less excusable and also more threatening.
I also think there may be some suspicions that some violent instigators might be acting at the urging of a foreign government - which is the domain of the FBI.
I think it's reasonable to help them in that effort - while also condemning the police for their role.
The death of ONE MAN, George Floyd, caused mass outrage. If a foreign directed agent is able to instigate a similar tragedy, I think that's very very important to nail down.