While I have no issue with police investigating lawbreakers, the public should know what the police are doing and how, so they can be confident that police are correctly following the law while executing their duty.
But perhaps I'm reading too much into that wording, and in fact some protesters who took photos thought the arson was over-reach, so they supplied the photos to the police.
https://www.mprnews.org/story/2019/02/07/google-location-pol...
If there's an obfuscation of the actual investigation here, I'd guess it's more a hiding of highly automated tools that process open source intelligence, probably by policy of the federal agency with the tool. Imagine a FBI/DEA/NSA tool that links and summarizes social media activity based on identities. So facial recognition + cell database -> trail of pictures to craft a link-narrative, without the work of having to dig much or do automated facial recognition. Tangentially I wouldn't be surprised if declaring "antifa" a "terrorist" organization is actually an internal dog whistle that blesses the use of XKEYSCORE etc against American citizens.