When protests start getting gassed, everyone panics, and looters start to take advantage of the chaos. This is a natural consequence of applying tear gas to large crowds. People at the front start doing everything they can to get away, people in the back start to flee, and in the chaos, looting starts.
If you want to prevent looting, don't hose down peaceful protests. When protesters aren't panicking, they can police themselves, and stop violence before it happens. There's no shortage of videos and anecdotes of protests actively stopping looters/instigators, because they don't want their protests to turn violent.
The first rule of policing is that the police are the public, and that the public is the police. The only difference is that one gets paid to do it full-time.
All this flies out the window when the grenades start flying.