Why would you assume they haven't committed a crime? The camouflaged cops are clearly targeting individuals, not kettling up whole groups of peaceful protesters. Also, police brutality doesn't excuse serious crimes. We can do something about both at the same time.
I don't presume guilt (this is America, after all), but I rather suspect the Federales have some reason to arrest some but not others, and that reason might be things like arson, assault, or other serious crimes.
I'm not arguing that the media have pumped some things up to get ratings, but it's undeniable there have been criminal acts in and around legitimate protesters, perpetrated by bad actors, and it's not unreasonable to think perhaps the Feds have some evidence of wrongdoing by virtue of the targeted nature of the arrests.