Because you are told or made to by others. You are lead to believe something that might or might not be true. You receive only the details you need to be enraged or outraged, without knowing all the facts about a particular situation or circumstance. Those who are telling you could be leading you astray, even without your knowledge, because you do not investigate the circumstance yourself. You simply go by what's on TV or in the Newspaper.
To lay judgement without facts is a mistake on each person's part.
I also disagree with the 'use' of White Americans. It insinuates that all of America is white and that isn't the case. He hasn't been president long enough to assess his work.