This is a frankly bizarre take. BLM is a protest aimed at the government (specifically the police apparatus). They already investigate gang criminality (your link specifically mentions that the Chicago PD is investigating). So as far as BLM is concerned, that’s the appropriate response and the government is doing its job? What is there to protest here, it’s not like gangs are going to go “oh shit look BLM is protesting, better stop all our criminal activities”. You should feel sympathetic to them not protesting this, because the protest would be heard by people who already agree and by people who don’t care (see your own “preaching to the choir”-argument).

BLM protests disproportionate police action usually aimed at black people, because it’s not how they think the government should act, nor do they react appropriately afterwards. That’s their pretty singular purpose.

Your argument boils down to a strange combination of whataboutism and a strange attempt at gatekeeping “standing up for the civil rights of minorities”, which I’m not sure if I’ve ever seen someone else attempt tbh.