The problem is when you say something without any qualifiers like "I've read somewhere that..." or "Some people say..." or "According to..." etc. the general understanding of your recipients is that you are certain of the information you are passing. If someone asks me what time it is, and I answer "Eight o'clock," they will assume I know the exact time (as opposed to the situation where I say "It's gotta be around eight o'clock by now," where they will know it's an approximation and it can't be completely relied on).

So if you are passing an information without any attempt to verify it and without any disclaimers that it's unverified, you are in misleading your recipients into thinking that they are presented with a fact, when are in fact presented with a suspicion or allegation. And what do you call that thing when someone talks in a way that makes other people believe false things? Lying.