I'm not a liberatarian nor am I a US citizen (Australian),
however ...
IF you let LEO's access group chats THEN you absolutely need civilian accountable honest transparent clear oversight to avoid mission creep.
The task isn't as clear cut as "known criminal is part of OC groups", there's a clear Sorities Paradox asking when does an OC group chat become a neighbourhood watch | knitting group chat.
If our criminal of interest is part of RealMafiaChat, PrepperLifeStyles, CamThePolice, GuideDogsForVets, etc. etc. et al. then how many of these should be surveilled .. and how many other group chat members should also be surveilled ?
Yes. Take down real criminals.
Yes, it's an actual problem that real innocents get swept up Kafka-esque for being unwittingly criminal adjacent.
What's the cost benefit number and trade off for actual innocent lives ruined?