Because "ALL these protests and movements are out to render us destitute or DEAD, especially BLM. We cant afford to let our guards down! This is our Thermopylae! This GUN in my fucking hands is the ONE thing between me and a casket! So you fucks better not TRY anything or I'll SHOOT!" <eye twitch>
At least, that's my guess as to the current rationale among law enforcement.