Those who believe that they "tell it like it is" are mistaken. The reality is that they often tell it the way they percieve it to be. Much of how life is isn't fact, but rather opinion, after all.
Also, these types think they tell it like it is because they don't see where people who disagree with them are coming from.
We're all entitled to our opinions, including people who think they "tell it like it is." But those with differing opinions on certain matters often have legitimate reasons for disagreeing with you as well.