But for some reason the moment the police are involved, that presumption is reversed. Why?
Because the testimony by police is treated as a proof more reliable than nearly any contrary evidence, short of very clear video.
When did the tear gas throwers show they were justified in throwing tear gas at the police?
Throwing tear gas back (important word) at the police when you feel it’s unjustified is an interesting question. Arguably such an act could be an act of self defense, but that gets very complicated when the police are involved.
What right do citizens have to defend themselves against what they feel is unlawful police violence? Against non-state actors, you have the right of self defense, obviously. Against state actors it is obviously complicated, since nominally they have a monopoly on the legal use of force, and we don’t want everyone with a valid warrant to feel justified in shooting at the police.
But to assume that there is no force that the state can apply that would not justify self defense is to accept a complete erosion of all civil liberties once the police are involved. Not all wrongs perpetuated by the police are amenable to court restitution; if you lose out on your right to peacefully assemble, then can a court give that back after the fact? What happens if the police apply lethal force without justification; you can’t sue to get your life back.
It is obvious that some right to self defense against state action exists, but exactly where the line is seems quite fuzzy to me.