Calling someone brainwashed...

Calling someone brainwashed isn't saying they are wrong. You can be brainwashed and be right, though more by luck than anything else.
After all, for brainwashing to work, the information transmitted doesn't need to be true, only believable!

History has shown that humans are very willing to believe what they are told, if it seems feasible to them. They tend to trust authority figures - those chosen to present ideas in the media.

This trusting nature can be used against them by those who may not be worthy of our trust.