If they got some violent counter-protestors/antifa thugs, then the police would have a reason to start cracking down. As usual, we'll get a lopsided arrest rate (like 1 antifa thug for every 10 protestors) and the antifa thugs will get a slap on the wrist while the protestor gets nailed to the wall. The government could use this gradually erode the number of protestors, drive off (literally) normies who aren't looking for a fight by painting it as a violent insurrection, and then use force to disperse the rest even if it takes a while.
Those trucks are expensive, and a group of black bloc faggots could cause a lot of damage. Sure, they'll get their ass kicked in the process, but it wouldn't be hard for the government to use those thugs to start the process of breaking the protest up.
Trump did, it only cost him his Twitter account for like 12 hours and all sorts of angry articles in the media about how Trump was threatening violence on peaceful protestors and the military saying they'd never follow Trump's unlawful orders. After that, Trump barely said shit.