1. That flag violates US Flag Code which is a federal law, although not truly binding in a legal sense. So these people are ok with violating laws.
2. It’s inherently racist. It was created as a knee jerk reaction by law enforcement (a job) in response to Black Lives Matter protests (a race of people). If you’re so so offended by BLM support then get a different job.
3. It’s being co-opted by white supremacists & extremist groups (see #2).

These are what come to my mind any time I see one.