Because of news reports like this:
Judith Gans, who studies immigration at the Udall Center for Studies in Public Policy at the University of Arizona, said that what social psychologists call self-serving perception bias seemed to be at play. Both sides in the immigration debate accept information that confirms their biases, she said, and discard, ignore or rationalize information that does not. There is no better example than the role of crime in Arizona’s tumultuous immigration debate.
This paragraph is in an article that is actually about how one side in the immigration debate ignores or rationalizes information that does not confirm their bias. In this case, the anti-immigration side ignores the fact that the undocumented population is decreasing, crime in Arizona is down, immigrants are less likely to commit crimes, and the federal government is deporting undocumented immigrants in record numbers. But if you read the above paragraph, you'd come away with the impression that both sides have been distorting reality to fit their agenda.
One side in the immigration debate has fueled anti-immigrant panic with harrowing anecdotes and dubious statistics. The reporter who wrote the above piece knows that, because that's the point of the piece itself. But the conventions of "objective" journalism mandate that he pretend both sides are being equally dishonest even though the weight of the empirical evidence lies entirely on one side! This is why people don't understand what's at stake in public-policy debates.
-- A. Serwer