I said "many" people think that. Take a quick perusal through a reddit thread on the topic and you'll see that "many" do indeed think that. Often, the sentiment is accompanied by "ACAB" or other such slogans.

Or, try to post on Facebook that you think a certain police officer might be innocent, that the cause of death might be an overdose, or that it's inappropriate to engage in hagiography of a drug-addicted, arrest-resisting violent criminal with a horrifying rap sheet, however he might have died. Those are all reasonable points, supported by at least some decent evidence or moral sensibility, but it would certainly be the end of any career you have in the United States.