> As long as the Feds were just cracking the heads of black protestors and snatching brown people up out of their beds, Trumpists were hard-core thin blue line.

The FBI are investigators, not traditional police; they aren't generally the ones people are talking about when they say "thin blue line". It wasn't the FBI cracking heads and "snatching brown people out of their beds." The phrase is a reaction to perceptions of widespread civic disorder and violent crime; it really has nothing to do with the FBI at all.