Is it because these cops are afraid they might miss a chance to actually beat/rubber bullet/tear gas some punks?
I base this on my totally unresearched opinion that there are a lot of cops who want to kill or maim someone at least once and they will take their shot on the margins because they may not get it again. When I watch the videos of cops shooting people you almost see the split second decision where they think "is this justified or am I threatened?...probably not...but it's close, shoot!...because it may never be this close again!"
In these demonstrations now they don't want to blow the opportunity to cave in some antifa heads....or lob a nice spiraling/smoking can of tear gas into the middle of a group of minorities because the demonstrations could stop tomorrow and then back to traffic cop duty and your partner will be bragging how he horse-trampled some weasel from Northwestern and you didn't shoot your shot.