You never know which way is up
Once upon a time, you could be sure that, if a band of guys clad all in black with masks concealing their faces kicked down your door, they were not law enforcement. Then it got to where if a band a guys clad all in black with masks concealing their faces kicked down your door, it WAS law enforcement. Now, you never know: the band of guys in black with guns and faces concealed kicking down your door might just be faking a role as law enforcement. And if they shoot the homeowner ... well, it's still pretty hard to tell.
One old time deputy here pointed out the foolish and risk involved: he said if you were going on a raid, you should want to be in full uniform, badge glittering, short haircut visible, everything to make sure the persons inside know you were police. Why, he wondered, would police dress up like criminals in a TV series?