Maybe "activists" who have very strong ideological beliefs but unknowingly present their preach as fact-based reports in a very tricky way that looks very objective. The public are misled to believe their distorted conclusions.

I've seen a lot of them. On some topics especially when audience don't have access to the ground truth, they(zesty activists disguised as objective journalists) are majority. This lead to a big gap between the perceptions on the same topic among different groups of audience and each group consider the other one are brainwashed.

The difference between activists and advertisers is: the later are knowingly try to sell something while the former thought they are telling truth which make the hidden activists more deceptive.