Why is it problematic if people can't distinguish between real news and propaganda?

The simple answer is that these people wont know whats real or not. If I was constantly watching a source putting out propaganda that, "People who drive Jeeps will try to hit you on the road," and I belived this, then I would always be paranoid while driving past a Jeep on the road. Even though, in reality, it's not true and they're driving like normal people.