IMO, the police wanted to stop the guy without exposing themselves the fire. They used what was there. I really doubt they were thinking of precedents. Maybe they should have. If they had brought in a 50 cal rifle and shot through the barrier, I am not sure if the precedent wouldn't be the same.
I just don't think this event is quite as significant as you think. The general use of explosives would require someone to decide that collateral damage is okay as long as the target is stopped which sometimes happens already. I just don't see it carrying over to explosives and armed drones just because of this event. If we are heading down that slippery slope, that ride already started and this didn't add anything significant. The criminal courts punishing police officers and civil suits punishing the administration are the two things that might hold law enforcement in check. I think the first has been eroded already. I don't know about the second.