It might seem like an academic point to make, but from what I have seen, there were no tanks used by the police in Ferguson. I’ve seen what looks like a re-purposed MRAP (mine resistant ambush protected vehicle) which was almost certainly surplus from our wars in Iraq and Afghanistan. There was another armored vehicle I couldn’t identify, and there was some sort of police command vehicle that was in a modified M113 APC. None of these are tanks.
Tanks have tracks, copious armor, and a large cannon.