Correction: not when their actions are inciting a riot.
Otherwise, yes! Yes, I agree!
People can't be arrested for what might happen. They can't be arrested for what they might do. They can be arrested for conspiracy to commit a crime, which is itself a crime, but that too must be proven.
The first insults out of their mouths could have justified their arrests for incitement to riot. (A court could still have found them technically innocent of the charges later.) But the police did not act.
Again, this goes back to the relationship the police have with the citizens in LaGrange. Apparently not a good one. It needs work.